MIT OpenCourseWare
http://ocw.mit.edu
21L.017 The Art of the Probable: Literature and Probability
Spring 2008
This content is in the public domain.
Charles Darwin
The Origin of Species by means of Natural Selection
or The Preservation of Favoured Races in the Struggle for Life
First Edition (1859)
(excerpts from the sixth edition, modified slightly for the sake of continuity)
Chapter three: Struggle for existence
[The aim of this volume is to throw some light on the origin of species–that mystery of
mysteries, as it has been called by one of our greatest philosophers. In the immense variety of
nature, we classify life by observing differences–gradual at the level of varieties, which shade
into one another as they subsist within a common type, and with differences more marked
between such types at the level of species. These differences, often amounting to a notable
discontinuity between one species and its nearest cogeners at the next level of classification, that
of genus, appear related to the way that members of a species are well adapted to their conditions
of life and give rise to questions about their origin. It may be doubted whether sudden and
considerable deviations of structure are ever permanently propagated in a state of nature. Almost every
part of every organic being is so beautifully related to its complex conditions of life that it seems as
improbable that any part should have been suddenly produced perfect, as that a complex machine should
have been invented by man in a perfect state.]
How have all those exquisite adaptations of one part of the organisation to another part, and to
the conditions of life and of one organic being to another being, been perfected? We see these
beautiful co-adaptations most plainly in the woodpecker and the mistletoe; and only a little less
plainly in the humblest parasite which clings to the hairs of a quadruped or feathers of a bird; in
the structure of the beetle which dives through the water; in the plumed seed which is wafted by
the gentlest breeze; in short, we see beautiful adaptations everywhere and in every part of the
organic world. Again, it may be asked, how is it that varieties, which I have called incipient
species, become ultimately converted into good and distinct species, which in most cases
obviously differ from each other far more than do the varieties of the same species? How do
those groups of species, which constitute what are called distinct genera and which differ from
each other more than do the species of the same genus, arise?
All these results follow from the struggle for life. Owing to this struggle, what I have called
individual variations, however slight and from whatever cause proceeding, if they be in any
degree profitable to the individuals of a species, in their infinitely complex relations to other
organic beings and to their physical conditions of life, will tend to the preservation of such
individuals, and will generally be inherited by the offspring. The offspring, also, will thus have a
better chance of surviving, for, of the many individuals of any species which are periodically
born, but a small number can survive. I have called this principle, by which each slight variation,
if useful, is preserved, by the term natural selection, in order to mark its relation to man's power
of selection. We have seen that man by selection can certainly produce great results, and can
adapt organic beings to his own uses, through the accumulation of slight but useful variations,
given to him by the hand of Nature. But Natural Selection, we shall hereafter see, is a power
incessantly ready for action, and is as immeasurably superior to man's feeble efforts, as the
works of Nature are to those of Art.
-1
We will now discuss in a little more detail the struggle for existence. Unless it be thoroughly
engrained in the mind, the whole economy of nature, with every fact on distribution, rarity,
abundance, extinction, and variation, will be dimly seen or quite misunderstood. We behold the
face of nature bright with gladness, we often see superabundance of food; we do not see or we
forget that the birds which are idly singing round us mostly live on insects or seeds, and are thus
constantly destroying life; or we forget how largely these songsters, or their eggs, or their
nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind, that, though
food may be now superabundant, it is not so at all seasons of each recurring year. I should
premise that I use “struggle for existence” in a large and metaphorical sense, including
dependence of one being on another, and including (which is more important) not only the life of
the individual, but success in leaving progeny. Two canine animals, in a time of dearth, may be
truly said to struggle with each other which shall get food and live. But a plant on the edge of a
desert is said to struggle for life against the drought, though more properly it should be said to be
dependent on the moisture. A plant which annually produces a thousand seeds, of which only
one of an average comes to maturity, may be more truly said to struggle with the plants of the
same and other kinds which already clothe the ground. The mistletoe is dependent on the apple
and a few other trees, but can only in a far-fetched sense be said to struggle with these trees, for,
if too many of these parasites grow on the same tree, it languishes and dies. But several seedling
mistletoes, growing close together on the same branch, may more truly be said to struggle with
each other. As the mistletoe is disseminated by birds, its existence depends on them; and it may
metaphorically be said to struggle with other fruit-bearing plants, in tempting the birds to devour
and thus disseminate its seeds. In these several senses, which pass into each other, I use for
convenience sake the general term of Struggle for Existence.
A struggle for existence inevitably follows from the high rate at which all organic beings tend to
increase. Every being, which during its natural lifetime produces several eggs or seeds, must
suffer destruction during some period of its life, and during some season or occasional year,
otherwise, on the principle of geometrical increase, its numbers would quickly become so
inordinately great that no country could support the product. Hence, as more individuals are
produced than can possibly survive, there must in every case be a struggle for existence, either
one individual with another of the same species, or with the individuals of distinct species, or
with the physical conditions of life. It is the doctrine of Malthus applied with manifold force to
the whole animal and vegetable kingdoms; for in this case there can be no artificial increase of
food, and no prudential restraint from marriage. Although some species may be now increasing,
more or less rapidly, in numbers, all cannot do so, for the world would not hold them. Linnaeus
has calculated that if an annual plant produced only two seeds--and there is no plant so
unproductive as this--and their seedlings next year produced two, and so on, then in twenty years
there would be a million plants. The elephant is reckoned the slowest breeder of all known
animals, and I have taken some pains to estimate its probable minimum rate of natural increase;
it will be safest to assume that it begins breeding when thirty years old, and goes on breeding till
ninety years old, bringing forth six young in the interval, and surviving till one hundred years
old; if this be so, after a period of from 740 to 750 years there would be nearly nineteen million
elephants alive descended from the first pair.
But we have better evidence on this subject than mere theoretical calculations, namely, the
numerous recorded cases of the astonishingly rapid increase of various animals in a state of
nature, when circumstances have been favourable to them during two or three following seasons.
-2
Still more striking is the evidence from our domestic animals of many kinds which have run wild
in several parts of the world; if the statements of the rate of increase of slow-breeding cattle and
horses in South America, and latterly in Australia, had not been well authenticated, they would
have been incredible. So it is with plants; cases could be given of introduced plants which have
become common throughout whole islands in a period of less than ten years. The only difference
between organisms which annually produce eggs or seeds by the thousand, and those which
produce extremely few, is, that the slow breeders would require a few more years to people,
under favourable conditions, a whole district, let it be ever so large. A large number of eggs is of
some importance to those species which depend on a fluctuating amount of food, for it allows
them rapidly to increase in number. But the real importance of a large number of eggs or seeds is
to make up for much destruction at some period of life; and this period in the great majority of
cases is an early one. If an animal can in any way protect its own eggs or young, a small number
may be produced, and yet the average stock be fully kept up; but if many eggs or young are
destroyed, many must be produced or the species will become extinct. It would suffice to keep
up the full number of a tree, which lived on an average for a thousand years, if a single seed were
produced once in a thousand years, supposing that this seed were never destroyed and could be
ensured to germinate in a fitting place; so that, in all cases, the average number of any animal or
plant depends only indirectly on the number of its eggs or seeds.
In looking at Nature, it is most necessary to keep the foregoing considerations always in
mind--never to forget that every single organic being may be said to be striving to the utmost to
increase in numbers; that each lives by a struggle at some period of its life; that heavy
destruction inevitably falls either on the young or old during each generation or at recurrent
intervals. Lighten any check, mitigate the destruction ever so little, and the number of the species
will almost instantaneously increase to any amount.
The causes which check the natural tendency of each species to increase are most obscure. Look
at the most vigorous species; by as much as it swarms in numbers, by so much will it tend to
increase still further. We know not exactly what the checks are even in a single instance. . . .
Climate plays an important part in determining the average numbers of a species, and periodical
seasons of extreme cold or drought seem to be the most effective of all checks. The action of
climate seems at first sight to be quite independent of the struggle for existence; but in so far as
climate chiefly acts in reducing food, it brings on the most severe struggle between the
individuals, whether of the same or of distinct species, which subsist on the same kind of food.
When we travel from south to north, or from a damp region to a dry, we invariably see some
species gradually getting rarer and rarer, and finally disappearing; and the change of climate
being conspicuous, we are tempted to attribute the whole effect to its direct action. But this is a
false view; we forget that each species, even where it most abounds, is constantly suffering
enormous destruction at some period of its life, from enemies or from competitors for the same
place and food; and if these enemies or competitors be in the least degree favoured by any slight
change of climate, they will increase in numbers; and as each area is already fully stocked with
inhabitants, the other species must decrease. When we travel southward and see a species
decreasing in numbers, we may feel sure that the cause lies quite as much in other species being
favoured, as in this one being hurt. It is only when we reach the Arctic regions, or snow-capped
summits, or absolute deserts, that the struggle for life is almost exclusively with the elements.
Many cases are on record showing how complex and unexpected are the checks and relations
-3
between organic beings, which have to struggle together in the same country. Near Farnham, in
Surrey, there are extensive heaths, with a few clumps of old Scotch firs on the distant hill-tops:
within the last ten years large spaces have been enclosed to keep out cattle, and self-sown firs are
now springing up in multitudes, so close together that all cannot live. When I ascertained that
these young trees had not been sown or planted I was so much surprised at their numbers that I
went to several points of view, whence I could examine hundreds of acres of the unenclosed
heath, and literally I could not see a single Scotch fir, except the old planted clumps. But on
looking closely between the stems of the heath, I found a multitude of seedlings and little trees,
which had been perpetually browsed down by the cattle. In one square yard, at a point some
hundred yards distant from one of the old clumps, I counted thirty-two little trees; and one of
them, with twenty-six rings of growth, had, during many years tried to raise its head above the
stems of the heath, and had failed. No wonder that, as soon as the land was enclosed, it became
thickly clothed with vigorously growing young firs. Yet the heath was so extremely barren and
so extensive that no one would ever have imagined that cattle would have so closely and
effectually searched it for food.
Here we see that cattle absolutely determine the existence of the Scotch fir; but in several parts
of the world insects determine the existence of cattle. Perhaps Paraguay offers the most curious
instance of this; for here neither cattle nor horses nor dogs have ever run wild, though they
swarm southward and northward in a feral state; and Azara and Rengger have shown that this is
caused by the greater number in Paraguay of a certain fly, which lays its eggs in the navels of
these animals when first born. The increase of these flies, numerous as they are, must be
habitually checked by some means, probably by other parasitic insects. Hence, if certain
insectivorous birds were to decrease in Paraguay, the parasitic insects would probably increase;
and this would lessen the number of the navel-frequenting flies--then cattle and horses would
become feral, and this would certainly greatly alter (as indeed I have observed in parts of South
America) the vegetation: this again would largely affect the insects; and this, as we have just
seen in Staffordshire, the insectivorous birds, and so onwards in ever-increasing circles of
complexity. Not that under nature the relations will ever be as simple as this. Battle within battle
must be continually recurring with varying success; and yet in the long-run the forces are so
nicely balanced that the face of nature remains for long periods of time uniform, though
assuredly the merest trifle would give the victory to one organic being over another.
Nevertheless, so profound is our ignorance, and so high our presumption, that we marvel when
we hear of the extinction of an organic being; and as we do not see the cause, we invoke
cataclysms to desolate the world, or invent laws on the duration of the forms of life!
I am tempted to give one more instance showing how plants and animals, remote in the scale of
nature, are bound together by a web of complex relations. I shall hereafter have occasion to show
that the exotic Lobelia fulgens is never visited in my garden by insects, and consequently, from
its peculiar structure, never sets a seed. Nearly all our orchidaceous plants absolutely require the
visits of insects to remove their pollen-masses and thus to fertilise them. I find from experiments
that humble-bees are almost indispensable to the fertilisation of the heartsease (Viola tricolor),
for other bees do not visit this flower. I have also found that the visits of bees are necessary for
the fertilisation of some kinds of clover; for instance twenty heads of Dutch clover (Trifolium
repens) yielded 2,290 seeds, but twenty other heads, protected from bees, produced not one.
Again, 100 heads of red clover (T. pratense) produced 2,700 seeds, but the same number of
protected heads produced not a single seed. Humble bees alone visit red clover, as other bees
-4
cannot reach the nectar. It has been suggested that moths may fertilise the clovers; but I doubt
whether they could do so in the case of the red clover, from their weight not being sufficient to
depress the wing petals. Hence we may infer as highly probable that, if the whole genus of
humble-bees became extinct or very rare in England, the heartsease and red clover would
become very rare, or wholly disappear. The number of humble-bees in any district depends in a
great measure upon the number of field-mice, which destroy their combs and nests; and Colonel
Newman, who has long attended to the habits of humble-bees, believes that "more than
two-thirds of them are thus destroyed all over England." Now the number of mice is largely
dependent, as every one knows, on the number of cats; and Colonel Newman says, "Near
villages and small towns I have found the nests of humble-bees more numerous than elsewhere,
which I attribute to the number of cats that destroy the mice." Hence it is quite credible that the
presence of a feline animal in large numbers in a district might determine, through the
intervention first of mice and then of bees, the frequency of certain flowers in that district!
In the case of every species, many different checks, acting at different periods of life, and during
different seasons or years, probably come into play; some one check or some few being
generally the most potent, but all will concur in determining the average number, or even the
existence of the species. In some cases it can be shown that widely-different checks act on the
same species in different districts. When we look at the plants and bushes clothing an entangled
bank, we are tempted to attribute their proportional numbers and kinds to what we call chance.
But how false a view is this! Every one has heard that when an American forest is cut down, a
very different vegetation springs up; but it has been observed that ancient Indian ruins in the
Southern United States, which must formerly have been cleared of trees, now display the same
beautiful diversity and proportion of kinds as in the surrounding virgin forests. What a struggle
must have gone on during long centuries between the several kinds of trees, each annually
scattering its seeds by the thousand; what war between insect and insect--between insects, snails,
and other animals with birds and beasts of prey--all striving to increase, all feeding on each
other, or on the trees, their seeds and seedlings, or on the other plants which first clothed the
ground and thus checked the growth of the trees. Throw up a handful of feathers, and all fall to
the ground according to definite laws; but how simple is the problem where each shall fall
compared to that of the action and reaction of the innumerable plants and animals which have
determined, in the course of centuries, the proportional numbers and kinds of trees now growing
on the old Indian ruins!
The dependency of one organic being on another, as of a parasite on its prey, lies generally
between beings remote in the scale of nature. This is likewise sometimes the case with those
which may strictly be said to struggle with each other for existence, as in the case of locusts and
grass-feeding quadrupeds. But the struggle will almost invariably be most severe between the
individuals of the same species, for they frequent the same districts, require the same food, and
are exposed to the same dangers. In the case of varieties of the same species, the struggle will
generally be almost equally severe, and we sometimes see the contest soon decided: for instance,
if several varieties of wheat be sown together, and the mixed seed be resown, some of the
varieties which best suit the soil or climate, or are naturally the most fertile, will beat the others
and so yield more seed, and will consequently in a few years supplant the other varieties.
As the species of the same genus usually have, though by no means invariably, much similarity
in habits and constitution, and always in structure, the struggle will generally be more severe
-5
between them, if they come into competition with each other, than between the species of
distinct genera. We see this in the recent extension over parts of the United States of one species
of swallow having caused the decrease of another species. The recent increase of the
missel-thrush in parts of Scotland has caused the decrease of the song-thrush. How frequently we
hear of one species of rat taking the place of another species under the most different climates!
We can dimly see why the competition should be most severe between allied forms, which fill
nearly the same place in the economy of nature; but probably in no one case could we precisely
say why one species has been victorious over another in the great battle of life.
A corollary of the highest importance may be deduced from the foregoing remarks, namely, that
the structure of every organic being is related, in the most essential yet often hidden manner, to
that of all other organic beings, with which it comes into competition for food or residence, or
from which it has to escape, or on which it preys. This is obvious in the structure of the teeth and
talons of the tiger; and in that of the legs and claws of the parasite which clings to the hair on the
tiger's body. But in the beautifully plumed seed of the dandelion, and in the flattened and fringed
legs of the water-beetle, the relation seems at first confined to the elements of air and water. Yet
the advantage of the plumed seeds no doubt stands in the closest relation to the land being
already thickly clothed with other plants; so that the seeds may be widely distributed and fall on
unoccupied ground. In the water-beetle, the structure of its legs, so well adapted for diving,
allows it to compete with other aquatic insects, to hunt for its own prey, and to escape serving as
prey to other animals.
Hence we can see that when a plant or animal is placed in a new country, among new
competitors, the conditions of its life will generally be changed in an essential manner, although
the climate may be exactly the same as in its former home. If its average numbers are to increase
in its new home, we should have to modify it in a different way to what we should have had to
do in its native country; for we should have to give it some advantage over a different set of
competitors or enemies. It is good thus to try in imagination to give any one species an
advantage over another. Probably in no single instance should we know what to do. This ought
to convince us of our ignorance on the mutual relations of all organic beings; a conviction as
necessary, as it is difficult to acquire. All that we can do is to keep steadily in mind that each
organic being is striving to increase in a geometrical ratio; that each, at some period of its life,
during some season of the year, during each generation, or at intervals, has to struggle for life
and to suffer great destruction. When we reflect on this struggle we may console ourselves with
the full belief that the war of nature is not incessant, that no fear is felt, that death is generally
prompt, and that the vigorous, the healthy, and the happy survive and multiply.
Chapter four: Natural Selection
How will the struggle for existence, briefly discussed in the last chapter, act in regard to
variation? Can the principle of selection, which we have seen is so potent in the hands of man,
apply under nature? I think we shall see that it can act most efficiently. Let the endless number
of slight variations and individual differences occurring in our domestic productions, and, in a
lesser degree, in those under nature, be borne in mind; as well as the strength of the hereditary
tendency. Under domestication, it may truly be said that the whole organisation becomes in some
degree plastic. Let it be borne in mind how infinitely complex and close-fitting are the mutual
relations of all organic beings to each other and to their physical conditions of life; and
-6
consequently what infinitely varied diversities of structure might be of use to each being under
changing conditions of life. Can it then be thought improbable, seeing that variations useful to
man have undoubtedly occurred, that other variations useful in some way to each being in the
great and complex battle of life, should occur in the course of many successive generations? If
such do occur, can we doubt (remembering that many more individuals are born than can
possibly survive) that individuals having any advantage, however slight, over others, would have
the best chance of surviving and procreating their kind? On the other hand, we may feel sure that
any variation in the least degree injurious would be rigidly destroyed. This preservation of
favourable individual differences and variations, and the destruction of those which are injurious,
I have called Natural Selection, or the Survival of the Fittest. Variations neither useful nor
injurious would not be affected by natural selection, and would be left either a fluctuating
element, as perhaps we see in certain polymorphic species, or would ultimately become fixed,
owing to the nature of the organism and the nature of the conditions.
Several writers have misapprehended or objected to the term Natural Selection. Some have even
imagined that natural selection induces variability, whereas it implies only the preservation of
such variations as arise and are beneficial to the being under its conditions of life. No one objects
to agriculturists speaking of the potent effects of man's selection; and in this case the individual
differences given by nature, which man for some object selects, must of necessity first occur.
Others have objected that the term selection implies conscious choice in the animals which
become modified; and it has even been urged that, as plants have no volition, natural selection is
not applicable to them! In the literal sense of the word, no doubt, natural selection is a false term;
but who ever objected to chemists speaking of an acid electing the base with which it in
preference combines. It has been said that I speak of natural selection as an active power or
Deity; but who objects to an author speaking of the attraction of gravity as ruling the movements
of the planets? So again it is difficult to avoid personifying the word Nature; but I mean by
nature, only the aggregate action and product of many natural laws, and by laws the sequence of
events as ascertained by us. With a little familiarity such superficial objections will be forgotten.
We shall best understand the probable course of natural selection by taking the case of a country
undergoing some slight physical change, for instance, of climate. The proportional numbers of
its inhabitants will almost immediately undergo a change, and some species will probably
become extinct. We may conclude, from what we have seen of the intimate and complex manner
in which the inhabitants of each country are bound together, that any change in the numerical
proportions of the inhabitants, independently of the change of climate itself, would seriously
affect the others. If the country were open on its borders, new forms would certainly immigrate,
and this would likewise seriously disturb the relations of some of the former inhabitants. But in
the case of an island, into which new and better adapted forms could not freely enter, we should
then have places in the economy of nature which would assuredly be better filled up if some of
the original inhabitants were in some manner modified. In such cases, slight modifications,
which in any way favoured the individuals of any species, by better adapting them to their
altered conditions, would tend to be preserved; and natural selection would have free scope for
the work of improvement.
We have reason to believe that changes in the conditions of life give a tendency to increased
variability; and in the foregoing cases the conditions the changed, and this would manifestly be
favourable to natural selection, by affording a better chance of the occurrence of profitable
-7
variations. Unless such occur, natural selection can do nothing. Under the term of "variations," it
must never be forgotten that individual differences are included. As man can produce a great
result with his domestic animals and plants by adding up in any given direction individual
differences [breeding up new varieties of plants or developing the greyhound and the lap-dog
from a common ancestral type], so could natural selection, but far more easily from having
incomparably longer time for action. Nor do I believe that any great physical change, as of
climate, or any unusual degree of isolation, to check immigration, is necessary in order that new
and unoccupied places should be left for natural selection to fill up by improving some of the
varying inhabitants. For as all the inhabitants of each country are struggling together with nicely
balanced forces, extremely slight modifications in the structure or habits of one species would
often give it an advantage over others; and still further modifications of the same kind would
often still further increase the advantage, as long as the species continued under the same
conditions of life and profited by similar means of subsistence and defence. No country can be
named in which all the native inhabitants are now so perfectly adapted to each other and to the
physical conditions under which they live, that none of them could be still better adapted or
improved; for in all countries, the natives have been so far conquered by naturalised productions
that they have allowed some foreigners to take firm possession of the land. And as foreigners
have thus in every country beaten some of the natives, we may safely conclude that the natives
might have been modified with advantage, so as to have better resisted the intruders.
[No breeder ever set out deliberately to produce a greyhound or a lapdog, but generations of
breeders have selected for size, never dreaming what the descendants would eventually become
through long-continued, partly unconscious and partly methodical selection.] Now, as man can
produce, and certainly has produced, a great result by his methodical and unconscious means of
selection, what may not natural selection effect? Man can act only on external and visible
characters: Nature cares nothing for appearances, except in so far as they are useful to any being.
She can act on every internal organ, on every shade of constitutional difference, on the whole
machinery of life. Man selects only for his own good; Nature only for that of the being which she
tends. Every selected character is fully exercised by her, as is implied by the fact of their
selection. Man keeps the natives of many climates in the same country. He seldom exercises
each selected character in some peculiar and fitting manner; he feeds a long and a short-beaked
pigeon on the same food; he does not exercise a long-backed or long-legged quadruped in any
peculiar manner; he exposes sheep with long and short wool to the same climate; does not allow
the most vigorous males to struggle for the females; he does not rigidly destroy all inferior
animals, but protects during each varying season, as far as lies in his power, all his productions.
He often begins his selection by some half-monstrous form, or at least by some modification
prominent enough to catch the eye or to be plainly useful to him. Under nature, the slightest
differences of structure or constitution may well turn the nicely-balanced scale in the struggle for
life, and so be preserved. How fleeting are the wishes and efforts of man! How short his time,
and consequently how poor will be his results, compared with those accumulated by Nature
during whole geological periods! Can we wonder, then, that Nature's productions should be far
"truer" in character than man's productions; that they should be infinitely better adapted to the
most complex conditions of life, and should plainly bear the stamp of far higher workmanship?
It may metaphorically be said that natural selection is daily and hourly scrutinising, throughout
the world, the slightest variations; rejecting those that are bad, preserving and adding up all that
are good; silently and insensibly working, WHENEVER AND WHEREVER OPPORTUNITY
-8
OFFERS, at the improvement of each organic being in relation to its organic and inorganic
conditions of life. We see nothing of these slow changes in progress, until the hand of time has
marked the long lapse of ages, and then so imperfect is our view into long-past geological ages
that we see only that the forms of life are now different from what they formerly were.
In order that any great amount of modification should be effected in a species, a variety, when
once formed must again, perhaps after a long interval of time, vary or present individual
differences of the same favourable nature as before; and these must again be preserved, and so
onward, step by step. Seeing that individual differences of the same kind perpetually recur, this
can hardly be considered as an unwarrantable assumption. But whether it is true, we can judge
only by seeing how far the hypothesis accords with and explains the general phenomena of
nature, for the ordinary belief that the amount of possible variation is a strictly limited quantity,
is likewise a simple assumption.
Although natural selection can act only through and for the good of each being, yet characters
and structures, which we are apt to consider as of very trifling importance, may thus be acted on.
When we see leaf-eating insects green, and bark-feeders mottled-grey; the alpine ptarmigan
white in winter, the red-grouse the colour of heather, we must believe that these tints are of
service to these birds and insects in preserving them from danger. Grouse, if not destroyed at
some period of their lives, would increase in countless numbers; they are known to suffer largely
from birds of prey; and hawks are guided by eyesight to their prey,--so much so that on parts of
the continent persons are warned not to keep white pigeons, as being the most liable to
destruction. Hence natural selection might be effective in giving the proper colour to each kind
of grouse, and in keeping that colour, when once acquired, true and constant. Nor ought we to
think that the occasional destruction of an animal of any particular colour would produce little
effect; we should remember how essential it is in a flock of white sheep to destroy a lamb with
the faintest trace of black. We have seen how the colour of hogs, which feed on the "paint-root"
in Virginia, determines whether they shall live or die. In plants, the down on the fruit and the
colour of the flesh are considered by botanists as characters of the most trifling importance; yet
we hear from an excellent horticulturist, Downing, that in the United States smooth-skinned
fruits suffer far more from a beetle, a Curculio, than those with down; that purple plums suffer
far more from a certain disease than yellow plums; whereas another disease attacks
yellow-fleshed peaches far more than those with other coloured flesh. If, with all the aids of art,
these slight differences make a great difference in cultivating the several varieties, assuredly, in a
state of nature, where the trees would have to struggle with other trees and with a host of
enemies, such differences would effectually settle which variety, whether a smooth or downy, a
yellow or a purple-fleshed fruit, should succeed. In looking at many small points of difference
between species, which, as far as our ignorance permits us to judge, seem quite unimportant, it is
also necessary to bear in mind that, owing to the law of correlation, when one part varies and the
variations are accumulated through natural selection, other modifications, often of the most
unexpected nature, will ensue.
As we see that those variations which, under domestication, appear at any particular period of
life, tend to reappear in the offspring at the same period; for instance, in the shape, size and
flavour of the seeds of the many varieties of our culinary and agricultural plants; in the
caterpillar and cocoon stages of the varieties of the silkworm; in the eggs of poultry, and in the
colour of the down of their chickens; in the horns of our sheep and cattle when nearly adult; so in
-9
a state of nature natural selection will be enabled to act on and modify organic beings at any age,
by the accumulation of variations profitable at that age, and by their inheritance at a
corresponding age. If it profit a plant to have its seeds more and more widely disseminated by
the wind, I can see no greater difficulty in this being effected through natural selection, than in
the cotton-planter increasing and improving by selection the down in the pods on his
cotton-trees. Natural selection may modify and adapt the larva of an insect to a score of
contingencies, wholly different from those which concern the mature insect; and these
modifications may affect, through correlation, the structure of the adult. So, conversely,
modifications in the adult may affect the structure of the larva; but in all cases natural selection
will ensure that they shall not be injurious: for if they were so, the species would become extinct.
Natural selection will modify the structure of the young in relation to the parent and of the parent
in relation to the young. In social animals it will adapt the structure of each individual for the
benefit of the whole community; if the community profits by the selected change. What natural
selection cannot do, is to modify the structure of one species, without giving it any advantage,
for the good of another species; and though statements to this effect may be found in works of
natural history, I cannot find one case which will bear investigation. A structure used only once
in an animal's life, if of high importance to it, might be modified to any extent by natural
selection; for instance, the great jaws possessed by certain insects, used exclusively for opening
the cocoon--or the hard tip to the beak of unhatched birds, used for breaking the eggs. It has been
asserted, that of the best short-beaked tumbler-pigeons a greater number perish in the egg than
are able to get out of it; so that fanciers assist in the act of hatching. Now, if nature had to make
the beak of a full-grown pigeon very short for the bird's own advantage, the process of
modification would be very slow, and there would be simultaneously the most rigorous selection
of all the young birds within the egg, which had the most powerful and hardest beaks, for all
with weak beaks would inevitably perish: or, more delicate and more easily broken shells might
be selected, the thickness of the shell being known to vary like every other structure.
SEXUAL SELECTION
Inasmuch as peculiarities often appear under domestication in one sex and become hereditarily
attached to that sex, so no doubt it will be under nature. Thus it is rendered possible for the two
sexes to be modified through natural selection in relation to different habits of life, as is
sometimes the case; or for one sex to be modified in relation to the other sex, as commonly
occurs. This leads me to say a few words on what I have called sexual selection. This form of
selection depends, not on a struggle for existence in relation to other organic beings or to
external conditions, but on a struggle between the individuals of one sex, generally the males, for
the possession of the other sex. The result is not death to the unsuccessful competitor, but few or
no offspring. Sexual selection is, therefore, less rigorous than natural selection. Generally, the
most vigorous males, those which are best fitted for their places in nature, will leave most
progeny. But in many cases victory depends not so much on general vigour, but on having
special weapons, confined to the male sex. A hornless stag or spurless cock would have a poor
chance of leaving numerous offspring. Sexual selection, by always allowing the victor to breed,
might surely give indomitable courage, length of spur, and strength to the wing to strike in the
spurred leg, in nearly the same manner as does the brutal cockfighter by the careful selection of
his best cocks. How low in the scale of nature the law of battle descends I know not; male
alligators have been described as fighting, bellowing, and whirling round, like Indians in a
-10
war-dance, for the possession of the females; male salmons have been observed fighting all day
long; male stag-beetles sometimes bear wounds from the huge mandibles of other males; the
males of certain hymenopterous insects have been frequently seen by that inimitable observer M.
Fabre, fighting for a particular female who sits by, an apparently unconcerned beholder of the
struggle, and then retires with the conqueror. The war is, perhaps, severest between the males of
polygamous animals, and these seem oftenest provided with special weapons. The males of
carnivorous animals are already well armed; though to them and to others, special means of
defence may be given through means of sexual selection, as the mane of the lion, and the hooked
jaw to the male salmon; for the shield may be as important for victory as the sword or spear.
Among birds, the contest is often of a more peaceful character. All those who have attended to
the subject, believe that there is the severest rivalry between the males of many species to attract,
by singing, the females. The rock-thrush of Guiana, birds of paradise, and some others,
congregate, and successive males display with the most elaborate care, and show off in the best
manner, their gorgeous plumage; they likewise perform strange antics before the females, which,
standing by as spectators, at last choose the most attractive partner. Those who have closely
attended to birds in confinement well know that they often take individual preferences and
dislikes: thus Sir R. Heron has described how a pied peacock was eminently attractive to all his
hen birds. I cannot here enter on the necessary details; but if man can in a short time give beauty
and an elegant carriage to his bantams, according to his standard of beauty, I can see no good
reason to doubt that female birds, by selecting, during thousands of generations, the most
melodious or beautiful males, according to their standard of beauty, might produce a marked
effect. Some well-known laws, with respect to the plumage of male and female birds, in
comparison with the plumage of the young, can partly be explained through the action of sexual
selection on variations occurring at different ages, and transmitted to the males alone or to both
sexes at corresponding ages; but I have not space here to enter on this subject.
Thus it is, as I believe, that when the males and females of any animal have the same general
habits of life, but differ in structure, colour, or ornament, such differences have been mainly
caused by sexual selection: that is, by individual males having had, in successive generations,
some slight advantage over other males, in their weapons, means of defence, or charms; which
they have transmitted to their male offspring alone. Yet, I would not wish to attribute all sexual
differences to this agency: for we see in our domestic animals peculiarities arising and becoming
attached to the male sex, which apparently have not been augmented through selection by man.
The tuft of hair on the breast of the wild turkey-cock cannot be of any use, and it is doubtful
whether it can be ornamental in the eyes of the female bird; indeed, had the tuft appeared under
domestication it would have been called a monstrosity.
NATURAL SELECTION, OR THE SURVIVAL OF THE FITTEST, ILLUSTRATED.
In order to make it clear how, as I believe, natural selection acts, I must beg permission to give
one or two imaginary illustrations. Let us take the case of a wolf, which preys on various
animals, securing some by craft, some by strength, and some by fleetness; and let us suppose that
the fleetest prey, a deer for instance, had from any change in the country increased in numbers,
or that other prey had decreased in numbers, during that season of the year when the wolf was
hardest pressed for food. Under such circumstances the swiftest and slimmest wolves have the
best chance of surviving, and so be preserved or selected, provided always that they retained
-11
strength to master their prey at this or some other period of the year, when they were compelled
to prey on other animals. I can see no more reason to doubt that this would be the result, than
that man should be able to improve the fleetness of his greyhounds by careful and methodical
selection, or by that kind of unconscious selection which follows from each man trying to keep
the best dogs without any thought of modifying the breed. I may add that, according to Mr.
Pierce, there are two varieties of the wolf inhabiting the Catskill Mountains, in the United States,
one with a light greyhound-like form, which pursues deer, and the other more bulky, with shorter
legs, which more frequently attacks the shepherd's flocks.
Even without any change in the proportional numbers of the animals on which our wolf preyed, a
cub might be born with an innate tendency to pursue certain kinds of prey, just as the tendency to
catch rats rather than mice is known to be inherited among our domestic felines. Now, if any
slight innate change of habit or of structure benefited an individual wolf, it would have the best
chance of surviving and of leaving offspring. Some of its young would probably inherit the same
habits or structure, and by the repetition of this process, a new variety might be formed which
would either supplant or coexist with the parent-form of wolf. Or, again, the wolves inhabiting a
mountainous district, and those frequenting the lowlands, would naturally be forced to hunt
different prey; and from the continued preservation of the individuals best fitted for the two sites,
two varieties might slowly be formed. These varieties would cross and blend where they met; but
to this subject of intercrossing we shall soon have to return.
It should be observed that in the above illustration, I speak of the slimmest individual wolves,
and not of any single strongly marked variation unique to a given wolf having been preserved.
To appreciate how rarely single variations, whether slight or strongly marked, could be
perpetuated, take the case of a pair of animals, producing during their lifetime two hundred
offspring, of which, from various causes of destruction, only two on an average survive to
pro-create their kind. Supposing that a single individual were born, which varied in some
manner, giving it twice as good a chance of life as that of the other individuals, yet the chances
would be strongly against its survival. Now, supposing nonetheless that it survives and breeds,
and that half its young inherited the favourable variation; still, the young would have only a
slightly better chance of surviving and breeding; and this chance would go on decreasing in the
succeeding generations. The justice of these remarks cannot, I think, be disputed. If, for instance,
a bird of some kind could procure its food more easily by having its beak curved, and if one were
born with its beak strongly curved, and which consequently flourished, nevertheless there would
be a very poor chance of this one individual perpetuating its kind to the exclusion of the common
form; but there can hardly be a doubt, judging by what we see taking place under domestication,
that this result would follow from the preservation during many generations of a large number of
individuals with more or less strongly curved beaks, and from the destruction of a still larger
number with the straightest beaks.
It may be worth while to give another and more complex illustration of the action of natural
selection. Certain plants excrete sweet juice, apparently for the sake of eliminating something
injurious from the sap: this is effected, for instance, by glands at the base of the stipules in some
Leguminosae, and at the backs of the leaves of the common laurel. This juice, though small in
quantity, is greedily sought by insects; but their visits do not in any way benefit the plant. Now,
let us suppose that the juice or nectar was excreted from the inside of the flowers of a certain
number of plants of any species. Insects in seeking the nectar would get dusted with pollen, and
-12
would often transport it from one flower to another. The flowers of two distinct individuals of
the same species would thus get crossed; and the act of crossing, as can be fully proved, gives
rise to vigorous seedlings, which consequently would have the best chance of flourishing and
surviving. The plants which produced flowers with the largest glands or nectaries, excreting
most nectar, would oftenest be visited by insects, and would oftenest be crossed; and so in the
long-run would gain the upper hand and form a local variety. The flowers, also, which had their
stamens and pistils placed, in relation to the size and habits of the particular insect which visited
them, so as to favour in any degree the transportal of the pollen, would likewise be favoured. We
might have taken the case of insects visiting flowers for the sake of collecting pollen instead of
nectar; and as pollen is formed for the sole purpose of fertilisation, its destruction appears to be a
simple loss to the plant; yet if a little pollen were carried, at first occasionally and then
habitually, by the pollen-devouring insects from flower to flower, and a cross thus effected,
although nine-tenths of the pollen were destroyed it might still be a great gain to the plant to be
thus robbed; and the individuals which produced more and more pollen, and had larger anthers,
would be selected.
When our plant, by the above process long continued, had been rendered highly attractive to
insects, they would, unintentionally on their part, regularly carry pollen from flower to flower;
and that they do this effectually I could easily show by many striking facts. I will give only one,
as likewise illustrating one step in the separation of the sexes of plants. Some holly-trees bear
only male flowers, which have four stamens producing a rather small quantity of pollen, and a
rudimentary pistil; other holly-trees bear only female flowers; these have a full-sized pistil, and
four stamens with shrivelled anthers, in which not a grain of pollen can be detected. Having
found a female tree exactly sixty yards from a male tree, I put the stigmas of twenty flowers,
taken from different branches, under the microscope, and on all, without exception, there were a
few pollen-grains, and on some a profusion. As the wind had set for several days from the female
to the male tree, the pollen could not thus have been carried. The weather had been cold and
boisterous and therefore not favourable to bees, nevertheless every female flower which I
examined had been effectually fertilised by the bees, which had flown from tree to tree in search
of nectar. But to return to our imaginary case; as soon as the plant had been rendered so highly
attractive to insects that pollen was regularly carried from flower to flower, another process
might commence. No naturalist doubts the advantage of what has been called the "physiological
division of labour;" hence we may believe that it would be advantageous to a plant to produce
stamens alone in one flower or on one whole plant, and pistils alone in another flower or on
another plant. In plants under culture and placed under new conditions of life, sometimes the
male organs and sometimes the female organs become more or less impotent; now if we suppose
this to occur in ever so slight a degree under nature, then, as pollen is already carried regularly
from flower to flower, and as a more complete separation of the sexes of our plant would be
advantageous on the principle of the division of labour, individuals with this tendency more and
more increased, would be continually favoured or selected, until at last a complete separation of
the sexes might be effected. It would take up too much space to show the various steps, through
dimorphism and other means, by which the separation of the sexes in plants of various kinds is
apparently now in progress; but I may add that some of the species of holly in North America
are, according to Asa Gray, in an exactly intermediate condition, or, as he expresses it, are more
or less dioeciously polygamous.
Let us now turn to the nectar-feeding insects; we may suppose the plant of which we have been
-13
slowly increasing the nectar by continued selection, to be a common plant; and that certain
insects depended in main part on its nectar for food. I could give many facts showing how
anxious bees are to save time: for instance, their habit of cutting holes and sucking the nectar at
the bases of certain flowers, which with a very little more trouble they can enter by the mouth.
Bearing such facts in mind, it may be believed that under certain circumstances individual
differences in the curvature or length of the proboscius and other parts, too slight to be
appreciated by us, might profit a bee or other insect, so that certain individuals would be able to
obtain their food more quickly than others; and thus the communities to which they belonged
would flourish and throw off many swarms inheriting the same peculiarities. The tubes of the
corolla of the common red or incarnate clovers (Trifolium pratense and incarnatum) do not on a
hasty glance appear to differ in length; yet the hive-bee can easily suck the nectar out of the
incarnate clover, but not out of the common red clover, which is visited by humble-bees alone;
so that whole fields of the red clover offer in vain an abundant supply of precious nectar to the
hive-bee. That this nectar is much liked by the hive-bee is certain; for I have repeatedly seen, but
only in the autumn, many hive-bees sucking the flowers through holes bitten in the base of the
tube by humble bees. The difference in the length of the corolla in the two kinds of clover, which
determines the visits of the hive-bee, must be very trifling; for I have been assured that when red
clover has been mown, the flowers of the second crop are somewhat smaller, and that these are
visited by many hive-bees. I do not know whether this statement is accurate; nor whether another
published statement can be trusted, namely, that the Ligurian bee, which is generally considered
a mere variety of the common hive-bee, and which freely crosses with it, is able to reach and
suck the nectar of the red clover. Thus, in a country where this kind of clover abounded, it might
be a great advantage to the hive-bee to have a slightly longer or differently constructed
proboscis. On the other hand, as the fertility of this clover absolutely depends on bees visiting
the flowers, if humble-bees were to become rare in any country, it might be a great advantage to
the plant to have a shorter or more deeply divided corolla, so that the hive-bees should be
enabled to suck its flowers. Thus I can understand how a flower and a bee might slowly become,
either simultaneously or one after the other, modified and adapted to each other in the most
perfect manner, by the continued preservation of all the individuals which presented slight
deviations of structure mutually favourable to each other.
I am well aware that this doctrine of natural selection, exemplified in the above imaginary
instances, is open to the same objections which were first urged against Sir Charles Lyell's noble
views on "the modern changes of the earth, as illustrative of geology;" but we now seldom hear
the agencies which we see still at work, spoken of as trifling and insignificant, when used in
explaining the excavation of the deepest valleys or the formation of long lines of inland cliffs.
Natural selection acts only by the preservation and accumulation of small inherited
modifications, each profitable to the preserved being; and as modern geology has almost
banished such views as the excavation of a great valley by a single diluvial wave, so will natural
selection banish the belief of the continued creation of new organic beings, or of any great and
sudden modification in their structure.
CIRCUMSTANCES FAVOURABLE FOR THE PRODUCTION OF NEW FORMS
THROUGH NATURAL SELECTION.
This is an extremely intricate subject. A great amount of variability, under which term individual
differences are always included, will evidently be favourable. A large number of individuals, by
-14
giving a better chance within any given period for the appearance of profitable variations, will
compensate for a lesser amount of variability in each individual, and is, I believe, a highly
important element of success. Though nature grants long periods of time for the work of natural
selection, she does not grant an indefinite period; for as all organic beings are striving to seize on
each place in the economy of nature, if any one species does not become modified and improved
in a corresponding degree with its competitors it will be exterminated. Unless favourable
variations be inherited by some at least of the offspring, nothing can be effected by natural
selection. The tendency to reversion may often check or prevent the work; but as this tendency
has not prevented man from forming by selection numerous domestic races, why should it
prevail against natural selection?
In the case of methodical selection, a breeder selects for some definite object, and if the
individuals be allowed freely to intercross, his work will completely fail. But when many men,
without intending to alter the breed, have a nearly common standard of perfection, and all try to
procure and breed from the best animals, improvement surely but slowly follows from this
unconscious process of selection, notwithstanding that there is no separation of selected
individuals. Thus it will be under nature; for within a confined area, with some place in the
natural polity not perfectly occupied, all the individuals varying in the right direction, though in
different degrees, will tend to be preserved. But if the area be large, its several districts will
almost certainly present different conditions of life; and then, if the same species undergoes
modification in different districts, the newly formed varieties will intercross on the confines of
each. But we shall see that intermediate varieties, inhabiting intermediate districts, will in the
long run generally be supplanted by one of the adjoining varieties. Intercrossing will chiefly
affect those animals which unite for each birth and wander much, and which do not breed at a
very quick rate. Hence with animals of this nature, for instance birds, varieties will generally be
confined to separated countries; and this I find to be the case. Even with animals which unite for
each birth, and which do not propagate rapidly, we must not assume that free intercrossing
would always eliminate the effects of natural selection; for I can bring forward a considerable
body of facts showing that within the same area two varieties of the same animal may long
remain distinct, from haunting different stations, from breeding at slightly different seasons, or
from the individuals of each variety preferring to pair together.
Intercrossing plays a very important part in nature by keeping the individuals of the same
species, or of the same variety, true and uniform in character. However, if the conditions of life
change and the form undergoes modification, uniformity of character can be given to the
modified offspring, solely by natural selection preserving similar favourable variations. Isolation
also is an important element in the modification of species through natural selection. In a
confined or isolated area, if not very large, the organic and inorganic conditions of life will
generally be almost uniform; so that natural selection will tend to modify all the varying
individuals of the same species in the same manner. Intercrossing with the inhabitants of the
surrounding districts, will also be thus prevented. Moritz Wagner has lately published an
interesting essay on this subject, and has shown that the service rendered by isolation in
preventing crosses between newly-formed varieties is probably greater even than I supposed. But
from reasons already assigned I can by no means agree with this naturalist, that migration and
isolation are necessary elements for the formation of new species. The importance of isolation is
likewise great in preventing, after any physical change in the conditions, such as of climate,
elevation of the land, etc., the immigration of better adapted organisms; and thus new places in
-15
the natural economy of the district will be left open to be filled up by the modification of the old
inhabitants. Lastly, isolation will give time for a new variety to be improved at a slow rate; and
this may sometimes be of much importance. If, however, an isolated area be very small, either
from being surrounded by barriers, or from having very peculiar physical conditions, the total
number of the inhabitants will be small; and this will retard the production of new species
through natural selection, by decreasing the chances of favourable variations arising.
The mere lapse of time by itself does nothing, either for or against natural selection. I state this
because it has been erroneously asserted that the element of time has been assumed by me to
play an all-important part in modifying species, as if all the forms of life were necessarily
undergoing change through some innate law. Lapse of time is only so far important, and its
importance in this respect is great, that it gives a better chance of beneficial variations arising
and of their being selected, accumulated, and fixed. It likewise tends to increase the direct action
of the physical conditions of life, in relation to the constitution of each organism.
Although isolation is of great importance in the production of new species, on the whole I am
inclined to believe that largeness of area is still more important, especially for the production of
species which shall prove capable of enduring for a long period, and of spreading widely.
Throughout a great and open area, not only will there be a better chance of favourable variations,
arising from the large number of individuals of the same species there supported, but the
conditions of life are much more complex from the large number of already existing species; and
if some of these many species become modified and improved, others will have to be improved
in a corresponding degree, or they will be exterminated. Each new form, also, as soon as it has
been much improved, will be able to spread over the open and continuous area, and will thus
come into competition with many other forms. Moreover, great areas, though now continuous,
will often, owing to former oscillations of level, have existed in a broken condition, so that the
good effects of isolation will generally, to a certain extent, have concurred. Finally, I conclude
that, although small isolated areas have been in some respects highly favourable for the
production of new species, yet that the course of modification will generally have been more
rapid on large areas; and what is more important, that the new forms produced on large areas,
which already have been victorious over many competitors, will be those that will spread most
widely, and will give rise to the greatest number of new varieties and species. They will thus
play a more important part in the changing history of the organic world.
In accordance with this view, we can, perhaps, understand, for instance, the fact of the
productions of the smaller continent of Australia now yielding before those of the larger
Europaeo-Asiatic area. Thus, also, it is that continental productions have everywhere become so
largely naturalised on islands. On a small island, the race for life will have been less severe, and
there will have been less modification and less extermination. Hence, we can understand how it
is that the flora of Madeira, resembles to a certain extent the extinct tertiary flora of Europe. All
fresh water basins, taken together, make a small area compared with that of the sea or of the
land. Consequently, the competition between fresh water productions will have been less severe
than elsewhere; new forms will have been more slowly produced, and old forms more slowly
exterminated. And it is in fresh water basins that we find seven genera of Ganoid fishes,
remnants of a once preponderant order: and in fresh water we find some of the most anomalous
forms now known in the world, as the Ornithorhynchus and Lepidosiren, which, like fossils,
connect to a certain extent orders at present widely separated in the natural scale. These
-16
anomalous forms may be called living fossils; they have endured to the present day, from having
inhabited a confined area, and from having been exposed to less varied, and therefore less
severe, competition.
To sum up, as far as the extreme intricacy of the subject permits, the circumstances favourable
and unfavourable for the production of new species through natural selection. I conclude that for
terrestrial productions a large continental area, which has undergone many oscillations of level,
will have been the most favourable for the production of many new forms of life, fitted to endure
for a long time and to spread widely. While the area existed as a continent the inhabitants will
have been numerous in individuals and kinds, and will have been subjected to severe
competition. When converted by subsidence into large separate islands there will still have
existed many individuals of the same species on each island: intercrossing on the confines of the
range of each new species will have been checked: after physical changes of any kind
immigration will have been prevented, so that new places in the polity of each island will have
had to be filled up by the modification of the old inhabitants; and time will have been allowed
for the varieties in each to become well modified and perfected. When, by renewed elevation, the
islands were reconverted into a continental area, there will again have been very severe
competition; the most favoured or improved varieties will have been enabled to spread; there
will have been much extinction of the less improved forms, and the relative proportional
numbers of the various inhabitants of the reunited continent will again have been changed; and
again there will have been a fair field for natural selection to improve still further the inhabitants,
and thus to produce new species.
That natural selection generally act with extreme slowness I fully admit. It can act only when
there are places in the natural polity of a district which can be better occupied by the
modification of some of its existing inhabitants. The occurrence of such places will often depend
on physical changes, which generally take place very slowly, and on the immigration of better
adapted forms being prevented. As some few of the old inhabitants become modified the mutual
relations of others will often be disturbed; and this will create new places, ready to be filled up
by better adapted forms; but all this will take place very slowly. Although all the individuals of
the same species differ in some slight degree from each other, it would often be long before
differences of the right nature in various parts of the organisation might occur. The result would
often be greatly retarded by free intercrossing. Many will exclaim that these several causes are
amply sufficient to neutralise the power of natural selection. I do not believe so. Slow though the
process of selection may be, if feeble man can do much by artificial selection, I can see no limit
to the amount of change, to the beauty and complexity of the coadaptations between all organic
beings, one with another and with their physical conditions of life, which may have been effected
in the long course of time through nature's power of selection, that is by the survival of the
fittest.
EXTINCTION CAUSED BY NATURAL SELECTION.
Owing to the high geometrical rate of increase of all organic beings, each area is already fully
stocked with inhabitants, and it follows from this, that as the favoured forms increase in number,
so, generally, will the less favoured decrease and become rare. Rarity, as geology tells us, is the
precursor to extinction. We can see that any form which is represented by few individuals will
run a good chance of utter extinction, during great fluctuations in the nature or the seasons, or
-17
from a temporary increase in the number of its enemies. But we may go further than this; for as
new forms are produced, unless we admit that specific forms can go on indefinitely increasing in
number, many old forms must become extinct. That the number of specific forms has not
indefinitely increased, geology plainly tells us; and we shall presently attempt to show why it is
that the number of species throughout the world has not become immeasurably great.
We have seen that the species which are most numerous in individuals have the best chance of
producing favourable variations within any given period. We have evidence of this, in the facts
stated in the second chapter, showing that it is the common and diffused or dominant species
which offer the greatest number of recorded varieties. Hence, rare species will be less quickly
modified or improved within any given period; they will consequently be beaten in the race for
life by the modified and improved descendants of the commoner species. We see the same
process of extermination among our domesticated productions, through the selection of
improved forms by man. Many curious instances could be given showing how quickly new
breeds of cattle, sheep and other animals, and varieties of flowers, take the place of older and
inferior kinds. In Yorkshire, it is historically known that the ancient black cattle were displaced
by the long-horns, and that these "were swept away by the short-horns" (I quote the words of an
agricultural writer) "as if by some murderous pestilence."
DIVERGENCE OF CHARACTER.
The principle, which I have designated by this term, is of high importance, and explains, as I
believe, several important facts. In the first place, varieties, even strongly-marked ones, though
having somewhat of the character of species--as is shown by the hopeless doubts in many cases
how to rank them--yet certainly differ far less from each other than do good and distinct species.
Nevertheless according to my view, varieties are species in the process of formation, or are, as I
have called them, incipient species. How, then, does the lesser difference between varieties
become augmented into the greater difference between species? That this does habitually
happen, we must infer from most of the innumerable species throughout nature presenting
well-marked differences; whereas varieties, the supposed prototypes and parents of future
well-marked species, present slight and ill-defined differences. Mere chance, as we may call it,
might cause one variety to differ in some character from its parents, and the offspring of this
variety again to differ from its parent in the very same character and in a greater degree; but this
alone would never account for so habitual and large a degree of difference as that between the
species of the same genus.
As has always been my practice, I have sought light on this head from our domestic productions.
We shall here find something analogous. It will be admitted that the production of races so
different as short-horn and Hereford cattle, race and cart horses, the several breeds of pigeons,
etc., could never have been effected by the mere chance accumulation of similar variations
during many successive generations. In practice, a fancier is, for instance, struck by a pigeon
having a slightly shorter beak; another fancier is struck by a pigeon having a rather longer beak;
and on the acknowledged principle that "fanciers do not and will not admire a medium standard,
but like extremes," they both go on (as has actually occurred with the sub-breeds of the
tumbler-pigeon) choosing and breeding from birds with longer and longer beaks, or with shorter
and shorter beaks. Again, we may suppose that at an early period of history, the men of one
nation or district required swifter horses, while those of another required stronger and bulkier
-18
horses. The early differences would be very slight; but, in the course of time, from the continued
selection of swifter horses in the one case, and of stronger ones in the other, the differences
would become greater, and would be noted as forming two sub-breeds. Ultimately after the lapse
of centuries, these sub-breeds would become converted into two well-established and distinct
breeds. As the differences became greater, the inferior animals with intermediate characters,
being neither very swift nor very strong, would not have been used for breeding, and will thus
have tended to disappear. Here, then, we see in man's productions the action of what may be
called the principle of divergence, causing differences, at first barely appreciable, steadily to
increase, and the breeds to diverge in character, both from each other and from their common
parent.
But how, it may be asked, can any analogous principle apply in nature? I believe it can and does
apply most efficiently (though it was a long time before I saw how), from the simple
circumstance that the more diversified the descendants from any one species become in
structure, constitution, and habits, by so much will they be better enabled to seize on many and
widely diversified places in the polity of nature, and so be enabled to increase in numbers.
We can clearly discern this in the case of animals with simple habits. Take the case of a
carnivorous quadruped, of which the number that can be supported in any country has long ago
arrived at its full average. If its natural power of increase be allowed to act, it can succeed in
increasing (the country not undergoing any change in conditions) only by its varying
descendants seizing on places at present occupied by other animals: some of them, for instance,
being enabled to feed on new kinds of prey, either dead or alive; some inhabiting new stations,
climbing trees, frequenting water, and some perhaps becoming less carnivorous. The more
diversified in habits and structure the descendants of our carnivorous animals become, the more
places they will be enabled to occupy. What applies to one animal will apply throughout all time
to all animals--that is, if they vary--for otherwise natural selection can effect nothing. So it will
be with plants. It has been experimentally proved, that if a plot of ground be sown with one
species of grass, and a similar plot be sown with several distinct genera of grasses, a greater
number of plants and a greater weight of dry herbage can be raised in the latter than in the
former case. The same has been found to hold good when one variety and several mixed varieties
of wheat have been sown on equal spaces of ground. Hence, if any one species of grass were to
go on varying, and the varieties were continually selected which differed from each other in the
same manner, though in a very slight degree, as do the distinct species and genera of grasses, a
greater number of individual plants of this species, including its modified descendants, would
succeed in living on the same piece of ground. And we know that each species and each variety
of grass is annually sowing almost countless seeds; and is thus striving, as it may be said, to the
utmost to increase in number. Consequently, in the course of many thousand generations, the
most distinct varieties of any one species of grass would have the best chance of succeeding and
of increasing in numbers, and thus of supplanting the less distinct varieties; and varieties, when
rendered very distinct from each other, take the rank of species.
The truth of the principle that the greatest amount of life can be supported by great
diversification of structure, is seen under many natural circumstances. In an extremely small
area, especially if freely open to immigration, and where the contest between individual and
individual must be very severe, we always find great diversity in its inhabitants. For instance, I
found that a piece of turf, three feet by four in size, which had been exposed for many years to
-19
exactly the same conditions, supported twenty species of plants, and these belonged to eighteen
genera and to eight orders, which shows how much these plants differed from each other. So it is
with the plants and insects on small and uniform islets: also in small ponds of fresh water.
Farmers find that they can raise more food by a rotation of plants belonging to the most different
orders: nature follows what may be called a simultaneous rotation. Most of the animals and
plants which live close round any small piece of ground, could live on it (supposing its nature
not to be in any way peculiar), and may be said to be striving to the utmost to live there; but, it is
seen, that where they come into the closest competition, the advantages of diversification of
structure, with the accompanying differences of habit and constitution, determine that the
inhabitants, which thus jostle each other most closely, shall, as a general rule, belong to what we
call different genera and orders.
The same principle is seen in the naturalisation of plants through man's agency in foreign lands.
It might have been expected that the plants which would succeed in becoming naturalised in any
land would generally have been closely allied to the indigenes; for these are commonly looked at
as specially created and adapted for their own country. It might also, perhaps, have been
expected that naturalised plants would have belonged to a few groups more especially adapted to
certain stations in their new homes. But the case is very different; and Alph. de Candolle has
well remarked, in his great and admirable work, that floras gain by naturalisation, proportionally
with the number of the native genera and species, far more in new genera than in new species.
To give a single instance: in the last edition of Dr. Asa Gray's "Manual of the Flora of the
Northern United States," 260 naturalised plants are enumerated, and these belong to 162 genera.
We thus see that these naturalised plants are of a highly diversified nature. They differ,
moreover, to a large extent, from the indigenes, for out of the 162 naturalised genera, no less
than 100 genera are not there indigenous, and thus a large proportional addition is made to the
genera now living in the United States.
By considering the nature of the plants or animals which have in any country struggled
successfully with the indigenes, and have there become naturalised, we may gain some crude
idea in what manner some of the natives would have had to be modified in order to gain an
advantage over their compatriots; and we may at least infer that diversification of structure,
amounting to new generic differences, would be profitable to them.
The advantage of diversification of structure in the inhabitants of the same region is, in fact, the
same as that of the physiological division of labour in the organs of the same individual body--a
subject so well elucidated by Milne Edwards. No physiologist doubts that a stomach by being
adapted to digest vegetable matter alone, or flesh alone, draws most nutriment from these
substances. So in the general economy of any land, the more widely and perfectly the animals
and plants are diversified for different habits of life, so will a greater number of individuals be
capable of there supporting themselves. A set of animals, with their organisation but little
diversified, could hardly compete with a set more perfectly diversified in structure. It may be
doubted, for instance, whether the Australian marsupials, which are divided into groups differing
but little from each other, and feebly representing, as Mr. Waterhouse and others have remarked,
our carnivorous, ruminant, and rodent mammals, could successfully compete with these
well-developed orders. In the Australian mammals, we see the process of diversification in an
early and incomplete stage of development. . . .
-20
ON THE DEGREE TO WHICH ORGANISATION TENDS TO ADVANCE.
Natural selection acts exclusively by the preservation and accumulation of variations, which are
beneficial under the organic and inorganic conditions to which each creature is exposed at all
periods of life. The ultimate result is that each creature tends to become more and more
improved in relation to its conditions. This improvement inevitably leads to the gradual
advancement of the organisation of the greater number of living beings throughout the world.
But here we enter on a very intricate subject, for naturalists have not defined to each other's
satisfaction what is meant by an advance in organisation. Among the vertebrata the degree of
intellect and an approach in structure to man clearly come into play. It might be thought that the
amount of change which the various parts and organs pass through in their development from
embryo to maturity would suffice as a standard of comparison; but there are cases, as with
certain parasitic crustaceans, in which several parts of the structure become less perfect, so that
the mature animal cannot be called higher than its larva. Von Baer's standard seems the most
widely applicable and the best, namely, the amount of differentiation of the parts of the same
organic being, in the adult state, as I should be inclined to add, and their specialisation for
different functions; or, as Milne Edwards would express it, the completeness of the division of
physiological labour. But we shall see how obscure this subject is if we look, for instance, to
fishes, among which some naturalists rank those as highest which, like the sharks, approach
nearest to amphibians; while other naturalists rank the common bony or teleostean fishes as the
highest, inasmuch as they are most strictly fish- like, and differ most from the other vertebrate
classes. We see still more plainly the obscurity of the subject by turning to plants, among which
the standard of intellect is of course quite excluded; and here some botanists rank those plants as
highest which have every organ, as sepals, petals, stamens and pistils, fully developed in each
flower; whereas other botanists, probably with more truth, look at the plants which have their
several organs much modified and reduced in number as the highest.
If we take as the standard of high organisation, the amount of differentiation and specialisation
of the several organs in each being when adult (and this will include the advancement of the
brain for intellectual purposes), natural selection clearly leads towards this standard: for all
physiologists admit that the specialisation of organs, inasmuch as in this state they perform their
functions better, is an advantage to each being; and hence the accumulation of variations tending
towards specialisation is within the scope of natural selection. On the other hand, we can see,
bearing in mind that all organic beings are striving to increase at a high ratio and to seize on
every unoccupied or less well occupied place in the economy of nature, that it is quite possible
for natural selection gradually to fit a being to a situation in which several organs would be
superfluous or useless: in such cases there would be retrogression in the scale of organisation.
Whether organisation on the whole has actually advanced from the remotest geological periods
to the present day will be more conveniently discussed in our chapter on Geological Succession.
But it may be objected that if all organic beings thus tend to rise in the scale, how is it that
throughout the world a multitude of the lowest forms still exist; and how is it that in each great
class some forms are far more highly developed than others? Why have not the more highly
developed forms every where supplanted and exterminated the lower? Lamarck, who believed in
an innate and inevitable tendency towards perfection in all organic beings, seems to have felt this
difficulty so strongly that he was led to suppose that new and simple forms are continually being
produced by spontaneous generation. Science has not as yet proved the truth of this belief,
-21
whatever the future may reveal. On our theory the continued existence of lowly organisms offers
no difficulty; for natural selection, or the survival of the fittest, does not necessarily include
progressive development--it only takes advantage of such variations as arise and are beneficial to
each creature under its complex relations of life. And it may be asked what advantage, as far as
we can see, would it be to an infusorian animalcule--to an intestinal worm--or even to an
earth-worm, to be highly organised. If it were no advantage, these forms would be left, by
natural selection, unimproved or but little improved, and might remain for indefinite ages in their
present lowly condition. And geology tells us that some of the lowest forms, as the infusoria and
rhizopods, have remained for an enormous period in nearly their present state. But to suppose
that most of the many now existing low forms have not in the least advanced since the first dawn
of life would be extremely rash; for every naturalist who has dissected some of the beings now
ranked as very low in the scale, must have been struck with their really wondrous and beautiful
organisation.
Nearly the same remarks are applicable, if we look to the different grades of organisation within
the same great group; for instance, in the vertebrata, to the co-existence of mammals and
fish--among mammalia, to the co-existence of man and the ornithorhynchus--among fishes, to
the co- existence of the shark and the lancelet (Amphioxus), which latter fish in the extreme
simplicity of its structure approaches the invertebrate classes. But mammals and fish hardly
come into competition with each other; the advancement of the whole class of mammals, or of
certain members in this class, to the highest grade would not lead to their taking the place of
fishes. Physiologists believe that the brain must be bathed by warm blood to be highly active,
and this requires aerial respiration; so that warm-blooded mammals when inhabiting the water lie
under a disadvantage in having to come continually to the surface to breathe. With fishes,
members of the shark family would not tend to supplant the lancelet; for the lancelet, as I hear
from Fritz Muller, has as sole companion and competitor on the barren sandy shore of South
Brazil, an anomalous annelid. The three lowest orders of mammals, namely, marsupials,
edentata, and rodents, co-exist in South America in the same region with numerous monkeys,
and probably interfere little with each other. Although organisation, on the whole, may have
advanced and be still advancing throughout the world, yet the scale will always present many
degrees of perfection; for the high advancement of certain whole classes, or of certain members
of each class, does not at all necessarily lead to the extinction of those groups with which they do
not enter into close competition. In some cases, as we shall hereafter see, lowly organised forms
appear to have been preserved to the present day, from inhabiting confined or peculiar stations,
where they have been subjected to less severe competition, and where their scanty numbers have
retarded the chance of favourable variations arising.
Finally, I believe that many lowly organised forms now exist throughout the world, from various
causes. In some cases variations or individual differences of a favourable nature may never have
arisen for natural selection to act on and accumulate. In no case, probably, has time sufficed for
the utmost possible amount of development. In some few cases there has been what we must call
retrogression or organisation. But the main cause lies in the fact that under very simple
conditions of life a high organisation would be of no service--possibly would be of actual
disservice, as being of a more delicate nature, and more liable to be put out of order and injured.
Looking to the first dawn of life, when all organic beings, as we may believe, presented the
simplest structure, how, it has been asked, could the first step in the advancement or
-22
differentiation of parts have arisen? As we have no facts to guide us, speculation on the subject
is almost useless. It is, however, an error to suppose that there would be no struggle for
existence, and, consequently, no natural selection, until many forms had been produced:
variations in a single species inhabiting an isolated station might be beneficial, and thus the
whole mass of individuals might be modified, or two distinct forms might arise.
CONVERGENCE OF CHARACTER.
Mr. H.C. Watson thinks that I have overrated the importance of divergence of character (in
which, however, he apparently believes), and that convergence, as it may be called, has likewise
played a part. If two species belonging to two distinct though allied genera, had both produced a
large number of new and divergent forms, it is conceivable that these might approach each other
so closely that they would have all to be classed under the same genus; and thus the descendants
of two distinct genera would converge into one. But it would in most cases be extremely rash to
attribute to convergence a close and general similarity of structure in the modified descendants
of widely distinct forms. The shape of a crystal is determined solely by the molecular forces, and
it is not surprising that dissimilar substances should sometimes assume the same form; but with
organic beings we should bear in mind that the form of each depends on an infinitude of complex
relations, namely on the variations which have arisen, these being due to causes far too intricate
to be followed out--on the nature of the variations which have been preserved or selected, and
this depends on the surrounding physical conditions, and in a still higher degree on the
surrounding organisms with which each being has come into competition--and lastly, on
inheritance (in itself a fluctuating element) from innumerable progenitors, all of which have had
their forms determined through equally complex relations. It is incredible that the descendants of
two organisms, which had originally differed in a marked manner, should ever afterwards
converge so closely as to lead to a near approach to identity throughout their whole organisation.
If this had occurred, we should meet with the same form, independently of genetic connection,
recurring in widely separated geological formations; and the balance of evidence is opposed to
any such an admission.
Mr. Watson has also objected that the continued action of natural selection, together with
divergence of character, would tend to make an indefinite number of specific forms. As far as
mere inorganic conditions are concerned, it seems probable that a sufficient number of species
would soon become adapted to all considerable diversities of heat, moisture, etc.; but I fully
admit that the mutual relations of organic beings are more important; and as the number of
species in any country goes on increasing, the organic conditions of life must become more and
more complex. Consequently there seems at first no limit to the amount of profitable
diversification of structure, and therefore no limit to the number of species which might be
produced. We do not know that even the most prolific area is fully stocked with specific forms:
at the Cape of Good Hope and in Australia, which support such an astonishing number of
species, many European plants have become naturalised. But geology shows us, that from an
early part of the tertiary period the number of species of shells, and that from the middle part of
this same period, the number of mammals has not greatly or at all increased. What then checks
an indefinite increase in the number of species? The amount of life (I do not mean the number of
specific forms) supported on an area must have a limit, depending so largely as it does on
physical conditions; therefore, if an area be inhabited by very many species, each or nearly each
species will be represented by few individuals; and such species will be liable to extermination
-23
from accidental fluctuations in the nature of the seasons or in the number of their enemies. The
process of extermination in such cases would be rapid, whereas the production of new species
must always be slow. Imagine the extreme case of as many species as individuals in England,
and the first severe winter or very dry summer would exterminate thousands on thousands of
species. Rare species, and each species will become rare if the number of species in any country
becomes indefinitely increased, will, on the principal often explained, present within a given
period few favourable variations; consequently, the process of giving birth to new specific forms
would thus be retarded. When any species becomes very rare, close interbreeding will help to
exterminate it; authors have thought that this comes into play in accounting for the deterioration
of the aurochs in Lithuania, of red deer in Scotland and of bears in Norway, etc. Lastly, and this I
am inclined to think is the most important element, a dominant species, which has already beaten
many competitors in its own home, will tend to spread and supplant many others. Alph. de
Candolle has shown that those species which spread widely tend generally to spread VERY
widely, consequently they will tend to supplant and exterminate several species in several areas,
and thus check the inordinate increase of specific forms throughout the world. Dr. Hooker has
recently shown that in the southeast corner of Australia, where, apparently, there are many
invaders from different quarters of the globe, the endemic Australian species have been greatly
reduced in number. How much weight to attribute to these several considerations I will not
pretend to say; but conjointly they must limit in each country the tendency to an indefinite
augmentation of specific forms.
SUMMARY OF CHAPTER.
If under changing conditions of life organic beings present individual differences in almost every
part of their structure, and this cannot be disputed; if there be, owing to their geometrical rate of
increase, a severe struggle for life at some age, season or year, and this certainly cannot be
disputed; then, considering the infinite complexity of the relations of all organic beings to each
other and to their conditions of life, causing an infinite diversity in structure, constitution, and
habits, to be advantageous to them, it would be a most extraordinary fact if no variations had
ever occurred useful to each being's own welfare, in the same manner as so many variations have
occurred useful to man. But if variations useful to any organic being ever do occur, assuredly
individuals thus characterised will have the best chance of being preserved in the struggle for
life; and from the strong principle of inheritance, these will tend to produce offspring similarly
characterised. This principle of preservation, or the survival of the fittest, I have called natural
selection. It leads to the improvement of each creature in relation to its organic and inorganic
conditions of life; and consequently, in most cases, to what must be regarded as an advance in
organisation. Nevertheless, low and simple forms will long endure if well fitted for their simple
conditions of life.
Whether natural selection has really thus acted in adapting the various forms of life to their
several conditions and stations, we have already seen how it entails extinction; and how largely
extinction has acted in the world's history, geology plainly declares. Natural selection, also, leads
to divergence of character; for the more organic beings diverge in structure, habits and
constitution, by so much the more can a large number be supported on the area, of which we see
proof by looking to the inhabitants of any small spot, and to the productions naturalised in
foreign lands. On these principles, the nature of the affinities, and the generally well defined
distinctions between the innumerable organic beings in each class throughout the world, may be
-24
explained. It is a truly wonderful fact--the wonder of which we are apt to overlook from
familiarity--that all animals and all plants throughout all time and space should be related to each
other in groups, subordinate to groups, in the manner which we everywhere behold--namely,
varieties of the same species most closely related, species of the same genus less closely and
unequally related, forming sections and sub-genera, species of distinct genera much less closely
related, and genera related in different degrees, forming sub-families, families, orders,
sub-classes, and classes. The several subordinate groups in any class cannot be ranked in a single
file, but seem clustered round points, and these round other points, and so on in almost endless
cycles. If species had been independently created, no explanation would have been possible of
this kind of classification; but it is explained through inheritance and the complex action of
natural selection, entailing extinction and divergence of character.
The affinities of all the beings of the same class have sometimes been represented by a great tree.
I believe this simile largely speaks the truth. The green and budding twigs may represent existing
species; and those produced during former years may represent the long succession of extinct
species. At each period of growth all the growing twigs have tried to branch out on all sides, and
to overtop and kill the surrounding twigs and branches, in the same manner as species and
groups of species have at all times overmastered other species in the great battle for life. The
limbs divided into great branches, and these into lesser and lesser branches, were themselves
once, when the tree was young, budding twigs; and this connexion of the former and present
buds by ramifying branches may well represent the classification of all extinct and living species
in groups subordinate to groups. Of the many twigs which flourished when the tree was a mere
bush, only two or three, now grown into great branches, yet survive and bear the other branches;
so with the species which lived during long-past geological periods, very few have left living and
modified descendants. From the first growth of the tree, many a limb and branch has decayed
and dropped off; and these fallen branches of various sizes may represent those whole orders,
families, and genera which have now no living representatives, and which are known to us only
in a fossil state. As we here and there see a thin, straggling branch springing from a fork low
down in a tree, and which by some chance has been favoured and is still alive on its summit, so
we occasionally see an animal like the Ornithorhynchus or Lepidosiren, which in some small
degree connects by its affinities two large branches of life, and which has apparently been saved
from fatal competition by having inhabited a protected station. As buds give rise by growth to
fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so
by generation I believe it has been with the great Tree of Life, which fills with its dead and
broken branches the crust of the earth, and covers the surface with its ever-branching and
beautiful ramifications.
Chapter six: Difficulties Of The Theory
Long before the reader has arrived at this part of my work, a crowd of difficulties will have
occurred to him. Some of them are so serious that to this day I can hardly reflect on them without
being in some degree staggered; but, to the best of my judgment, the greater number are only
apparent, and those that are real are not, I think, fatal to the theory.
ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.
As natural selection acts solely by the preservation of profitable modifications, each new form
-25
will tend in a fully-stocked country to take the place of, and finally to exterminate, its own less
improved parent-form and other less-favoured forms with which it comes into competition. Thus
extinction and natural selection go hand in hand. Hence, if we look at each species as descended
from some unknown form, both the parent and all the transitional varieties will generally have
been exterminated by the very process of the formation and perfection of the new form.
But, as by this theory innumerable transitional forms must have existed, why do we not find
them embedded in countless numbers in the crust of the earth? It will be more convenient to
discuss this question in the chapter on the imperfection of the geological record; and I will here
only state that I believe the answer mainly lies in the record being incomparably less perfect than
is generally supposed. The crust of the earth is a vast museum; but the natural collections have
been imperfectly made, and only at long intervals of time.
But it may be urged that when several closely allied species inhabit the same territory, we surely
ought to find at the present time many transitional forms. Let us take a simple case: in travelling
from north to south over a continent, we generally meet at successive intervals with closely
allied or representative species, evidently filling nearly the same place in the natural economy of
the land. These representative species often meet and interlock; and as the one becomes rarer and
rarer, the other becomes more and more frequent, till the one replaces the other. But if we
compare these species where they intermingle, they are generally as absolutely distinct from
each other in every detail of structure as are specimens taken from the metropolis inhabited by
each. By my theory these allied species are descended from a common parent; and during the
process of modification, each has become adapted to the conditions of life of its own region, and
has supplanted and exterminated its original parent-form and all the transitional varieties
between its past and present states. Hence we ought not to expect at the present time to meet
with numerous transitional varieties in each region, though they must have existed there, and
may be embedded there in a fossil condition. But in the intermediate region, having intermediate
conditions of life, why do we not now find closely-linking intermediate varieties? This difficulty
for a long time quite confounded me. But I think it can be in large part explained.
In the first place we should be extremely cautious in inferring, because an area is now
continuous, that it has been continuous during a long period. Geology would lead us to believe
that most continents have been broken up into islands even during the later tertiary periods; and
in such islands distinct species might have been separately formed without the possibility of
intermediate varieties existing in the intermediate zones. By changes in the form of the land and
of climate, marine areas now continuous must often have existed within recent times in a far less
continuous and uniform condition than at present. But I will pass over this way of escaping from
the difficulty; for I believe that many perfectly defined species have been formed on strictly
continuous areas; though I do not doubt that the formerly broken condition of areas now
continuous, has played an important part in the formation of new species, more especially with
freely-crossing and wandering animals.
In looking at species as they are now distributed over a wide area, we generally find them
tolerably numerous over a large territory, then becoming somewhat abruptly rarer and rarer on
the confines, and finally disappearing. Hence the neutral territory between two representative
species is generally narrow in comparison with the territory proper to each. We see the same fact
in ascending mountains, and sometimes it is quite remarkable how abruptly, as Alph. De
-26
Candolle has observed, a common alpine species disappears. The same fact has been noticed by
E. Forbes in sounding the depths of the sea with the dredge. To those who look at climate and
the physical conditions of life as the all-important elements of distribution, these facts ought to
cause surprise, as climate and height or depth graduate away insensibly. But when we bear in
mind that almost every species, even in its metropolis, would increase immensely in numbers,
were it not for other competing species; that nearly all either prey on or serve as prey for others;
in short, that each organic being is either directly or indirectly related in the most important
manner to other organic beings--we see that the range of the inhabitants of any country by no
means exclusively depends on insensibly changing physical conditions, but in large part on the
presence of other species, on which it lives, or by which it is destroyed, or with which it comes
into competition; and as these species are already defined objects, not blending one into another
by insensible gradations, the range of any one species, depending as it does on the range of
others, will tend to be sharply defined. Moreover, each species on the confines of its range,
where it exists in lessened numbers, will, during fluctuations in the number of its enemies or of
its prey, or in the nature of the seasons, be extremely liable to utter extermination; and thus its
geographical range will come to be still more sharply defined.
As allied or representative species, when inhabiting a continuous area, are generally distributed
in such a manner that each has a wide range, with a comparatively narrow neutral territory
between them, in which they become rather suddenly rarer and rarer; then, as varieties do not
essentially differ from species, the same rule will probably apply to both; and if we take a
varying species inhabiting a very large area, we shall have to adapt two varieties to two large
areas, and a third variety to a narrow intermediate zone. The intermediate variety, consequently,
will exist in lesser numbers from inhabiting a narrow and lesser area; and practically, as far as I
can make out, this rule holds good with varieties in a state of nature. I have met with striking
instances of the rule in the case of varieties intermediate between well-marked varieties in the
genus Balanus. And it would appear from information given me by Mr. Watson, Dr. Asa Gray,
and Mr. Wollaston, that generally, when varieties intermediate between two other forms occur,
they are much rarer numerically than the forms which they connect. Now, if we may trust these
facts and inferences, and conclude that varieties linking two other varieties together generally
have existed in lesser numbers than the forms which they connect, then we can understand why
intermediate varieties should not endure for very long periods: why, as a general rule, they
should be exterminated and disappear, sooner than the forms which they originally linked
together.
For any form existing in lesser numbers would, as already remarked, run a greater chance of
being exterminated than one existing in large numbers; and in this particular case the
intermediate form would be eminently liable to the inroads of closely allied forms existing on
both sides of it. But it is a far more important consideration, that during the process of further
modification, by which two varieties are supposed to be converted and perfected into two
distinct species, the two which exist in larger numbers, from inhabiting larger areas, will have a
great advantage over the intermediate variety, which exists in smaller numbers in a narrow and
intermediate zone. For forms existing in larger numbers will have a better chance, within any
given period, of presenting further favourable variations for natural selection to seize on, than
will the rarer forms which exist in lesser numbers. Hence, the more common forms, in the race
for life, will tend to beat and supplant the less common forms, for these will be more slowly
modified and improved. It is the same principle which, as I believe, accounts for the common
-27
species in each country, as shown in the second chapter, presenting on an average a greater
number of well-marked varieties than do the rarer species. I may illustrate what I mean by
supposing three varieties of sheep to be kept, one adapted to an extensive mountainous region; a
second to a comparatively narrow, hilly tract; and a third to the wide plains at the base; and that
the inhabitants are all trying with equal steadiness and skill to improve their stocks by selection;
the chances in this case will be strongly in favour of the great holders on the mountains or on the
plains improving their breeds more quickly than the small holders on the intermediate narrow,
hilly tract; and consequently the improved mountain or plain breed will soon take the place of
the less improved hill breed; and thus the two breeds, which originally existed in greater
numbers, will come into close contact with each other, without the interposition of the
supplanted, intermediate hill variety.
To sum up, I believe that species come to be tolerably well-defined objects, and do not at any
one period present an inextricable chaos of varying and intermediate links: first, because new
varieties are very slowly formed, for variation is a slow process, and natural selection can do
nothing until favourable individual differences or variations occur, and until a place in the
natural polity of the country can be better filled by some modification of some one or more of its
inhabitants. And such new places will depend on slow changes of climate, or on the occasional
immigration of new inhabitants, and, probably, in a still more important degree, on some of the
old inhabitants becoming slowly modified, with the new forms thus produced and the old ones
acting and reacting on each other. So that, in any one region and at any one time, we ought to see
only a few species presenting slight modifications of structure in some degree permanent; and
this assuredly we do see.
Secondly, areas now continuous must often have existed within the recent period as isolated
portions, in which many forms, more especially among the classes which unite for each birth and
wander much, may have separately been rendered sufficiently distinct to rank as representative
species. In this case, intermediate varieties between the several representative species and their
common parent, must formerly have existed within each isolated portion of the land, but these
links during the process of natural selection will have been supplanted and exterminated, so that
they will no longer be found in a living state.
Thirdly, when two or more varieties have been formed in different portions of a strictly
continuous area, intermediate varieties will, it is probable, at first have been formed in the
intermediate zones, but they will generally have had a short duration. For these intermediate
varieties will, from reasons already assigned (namely from what we know of the actual
distribution of closely allied or representative species, and likewise of acknowledged varieties),
exist in the intermediate zones in lesser numbers than the varieties which they tend to connect.
From this cause alone the intermediate varieties will be liable to accidental extermination; and
during the process of further modification through natural selection, they will almost certainly be
beaten and supplanted by the forms which they connect; for these, from existing in greater
numbers will, in the aggregate, present more varieties, and thus be further improved through
natural selection and gain further advantages.
Lastly, looking not to any one time, but at all time, if my theory be true, numberless intermediate
varieties, linking closely together all the species of the same group, must assuredly have existed;
but the very process of natural selection constantly tends, as has been so often remarked, to
-28
exterminate the parent forms and the intermediate links. Consequently evidence of their former
existence could be found among fossil remains, which are preserved, as we shall attempt to show
in a future chapter, in an extremely imperfect and intermittent record.
ON THE ORIGIN AND TRANSITION OF ORGANIC BEINGS WITH PECULIAR HABITS
AND STRUCTURE.
It has been asked by the opponents of such views as I hold, how, for instance, could a land
carnivorous animal have been converted into one with aquatic habits; for how could the animal
in its transitional state have subsisted? It would be easy to show that there now exist carnivorous
animals presenting close intermediate grades from strictly terrestrial to aquatic habits; and as
each exists by a struggle for life, it is clear that each must be well adapted to its place in nature.
Look at the Mustela vison of North America, which has webbed feet, and which resembles an
otter in its fur, short legs, and form of tail; during summer this animal dives for and preys on
fish, but during the long winter it leaves the frozen waters, and preys, like other polecats on mice
and land animals. If a different case had been taken, and it had been asked how an insectivorous
quadruped could possibly have been converted into a flying bat, the question would have been
far more difficult to answer. Yet I think such difficulties have little weight.
Here, as on other occasions, I lie under a heavy disadvantage, for, out of the many striking cases
which I have collected, I can give only one or two instances of transitional habits and structures
in allied species; and of diversified habits, either constant or occasional, in the same species. And
it seems to me that nothing less than a long list of such cases is sufficient to lessen the difficulty
in any particular case like that of the bat.
Look at the family of squirrels; here we have the finest gradation from animals with their tails
only slightly flattened, and from others, as Sir J. Richardson has remarked, with the posterior
part of their bodies rather wide and with the skin on their flanks rather full, to the so-called
flying squirrels; and flying squirrels have their limbs and even the base of the tail united by a
broad expanse of skin, which serves as a parachute and allows them to glide through the air to an
astonishing distance from tree to tree. We cannot doubt that each structure is of use to each kind
of squirrel in its own country, by enabling it to escape birds or beasts of prey, or to collect food
more quickly, or, as there is reason to believe, to lessen the danger from occasional falls. But it
does not follow from this fact that the structure of each squirrel is the best that it is possible to
conceive under all possible conditions. Let the climate and vegetation change, let other
competing rodents or new beasts of prey immigrate, or old ones become modified, and all
analogy would lead us to believe that some, at least, of the squirrels would decrease in numbers
or become exterminated, unless they also become modified and improved in structure in a
corresponding manner. Therefore, I can see no difficulty, more especially under changing
conditions of life, in the continued preservation of individuals with fuller and fuller
flank-membranes, each modification being useful, each being propagated, until, by the
accumulated effects of this process of natural selection, a perfect so-called flying squirrel was
produced.
Now look at the Galeopithecus or so-called flying lemur, which was formerly ranked among
bats, but is now believed to belong to the Insectivora. An extremely wide flank-membrane
stretches from the corners of the jaw to the tail, and includes the limbs with the elongated
-29
fingers. This flank- membrane is furnished with an extensor muscle. Although no graduated links
of structure, fitted for gliding through the air, now connect the Galeopithecus with the other
Insectivora, yet there is no difficulty in supposing that such links formerly existed, and that each
was developed in the same manner as with the less perfectly gliding squirrels; each grade of
structure having been useful to its possessor. Nor can I see any insuperable difficulty in further
believing it possible that the membrane-connected fingers and fore-arm of the Galeopithecus
might have been greatly lengthened by natural selection; and this, as far as the organs of flight
are concerned, would have converted the animal into a bat. In certain bats in which the
wing-membrane extends from the top of the shoulder to the tail and includes the hind-legs, we
perhaps see traces of an apparatus originally fitted for gliding through the air rather than for
flight.
If about a dozen genera of birds were to become extinct, who would have ventured to surmise
that birds might have existed which used their wings solely as flappers, like the logger headed
duck (Micropterus of Eyton); as fins in the water and as front legs on the land, like the penguin;
as sails, like the ostrich; and functionally for no purpose, like the apteryx? Yet the structure of
each of these birds is good for it, under the conditions of life to which it is exposed, for each has
to live by a struggle: but it is not necessarily the best possible under all possible conditions. It
must not be inferred from these remarks that any of the grades of wing-structure here alluded to,
which perhaps may all be the result of disuse, indicate the steps by which birds actually acquired
their perfect power of flight; but they serve to show what diversified means of transition are at
least possible.
Seeing that a few members of such water-breathing classes as the Crustacea and Mollusca are
adapted to live on the land; and seeing that we have flying birds and mammals, flying insects of
the most diversified types, and formerly had flying reptiles, it is conceivable that flying-fish,
which now glide far through the air, slightly rising and turning by the aid of their fluttering fins,
might have been modified into perfectly winged animals. If this had been effected, who would
have ever imagined that in an early transitional state they had been inhabitants of the open ocean,
and had used their incipient organs of flight exclusively, so far as we know, to escape being
devoured by other fish?
When we see any structure highly perfected for any particular habit, as the wings of a bird for
flight, we should bear in mind that animals displaying early transitional grades of the structure
will seldom have survived to the present day, for they will have been supplanted by their
successors, which were gradually rendered more perfect through natural selection. Furthermore,
we may conclude that transitional states between structures fitted for very different habits of life
will rarely have been developed at an early period in great numbers and under many subordinate
forms. Thus, to return to our imaginary illustration of the flying-fish, it does not seem probable
that fishes capable of true flight would have been developed under many subordinate forms, for
taking prey of many kinds in many ways, on the land and in the water, until their organs of flight
had come to a high stage of perfection, so as to have given them a decided advantage over other
animals in the battle for life. Hence the chance of discovering species with transitional grades of
structure in a fossil condition will always be less, from their having existed in lesser numbers,
than in the case of species with fully developed structures.
I will now give two or three instances, both of diversified and of changed habits, in the
-30
individuals of the same species. In either case it would be easy for natural selection to adapt the
structure of the animal to its changed habits, or exclusively to one of its several habits. It is,
however, difficult to decide and immaterial for us, whether habits generally change first and
structure afterwards; or whether slight modifications of structure lead to changed habits; both
probably often occurring almost simultaneously. Of cases of changed habits it will suffice
merely to allude to that of the many British insects which now feed on exotic plants, or
exclusively on artificial substances. Of diversified habits innumerable instances could be given: I
have often watched a tyrant flycatcher (Saurophagus sulphuratus) in South America, hovering
over one spot and then proceeding to another, like a kestrel, and at other times standing
stationary on the margin of water, and then dashing into it like a kingfisher at a fish. In our own
country the larger titmouse (Parus major) may be seen climbing branches, almost like a creeper;
it sometimes, like a shrike, kills small birds by blows on the head; and I have many times seen
and heard it hammering the seeds of the yew on a branch, and thus breaking them like a
nuthatch. In North America the black bear was seen by Hearne swimming for hours with widely
open mouth, thus catching, almost like a whale, insects in the water.
As we sometimes see individuals following habits different from those proper to their species
and to the other species of the same genus, we might expect that such individuals would
occasionally give rise to new species, having anomalous habits, and with their structure either
slightly or considerably modified from that of their type. And such instances occur in nature.
Can a more striking instance of adaptation be given than that of a woodpecker for climbing trees
and seizing insects in the chinks of the bark? Yet in North America there are woodpeckers which
feed largely on fruit, and others with elongated wings which chase insects on the wing. On the
plains of La Plata, where hardly a tree grows, there is a woodpecker (Colaptes campestris) which
has two toes before and two behind, a long- pointed tongue, pointed tail-feathers, sufficiently
stiff to support the bird in a vertical position on a post, but not so stiff as in the typical
wood-peckers, and a straight, strong beak. The beak, however, is not so straight or so strong as in
the typical woodpeckers but it is strong enough to bore into wood. Hence this Colaptes, in all the
essential parts of its structure, is a woodpecker. Even in such trifling characters as the colouring,
the harsh tone of the voice, and undulatory flight, its close blood-relationship to our common
woodpecker is plainly declared; yet, as I can assert, not only from my own observations, but
from those of the accurate Azara, in certain large districts it does not climb trees, and it makes its
nest in holes in banks! In certain other districts, however, this same woodpecker, as Mr. Hudson
states, frequents trees, and bores holes in the trunk for its nest. I may mention as another
illustration of the varied habits of this genus, that a Mexican Colaptes has been described by De
Saussure as boring holes into hard wood in order to lay up a store of acorns.
Petrels are the most aerial and oceanic of birds, but, in the quiet sounds of Tierra del Fuego, the
Puffinuria berardi, in its general habits, in its astonishing power of diving, in its manner of
swimming and of flying when made to take flight, would be mistaken by any one for an auk or a
grebe; nevertheless, it is essentially a petrel, but with many parts of its organisation profoundly
modified in relation to its new habits of life; whereas the woodpecker of La Plata has had its
structure only slightly modified. In the case of the water-ouzel, the acutest observer, by
examining its dead body, would never have suspected its sub-aquatic habits; yet this bird, which
is allied to the thrush family, subsists by diving,--using its wings under water and grasping
stones with its feet. All the members of the great order of Hymenopterous insects are terrestrial,
excepting the genus Proctotrupes, which Sir John Lubbock has discovered to be aquatic in its
-31
habits; it often enters the water and dives about by the use not of its legs but of its wings, and
remains as long as four hours beneath the surface; yet it exhibits no modification in structure in
accordance with its abnormal habits.
He who believes that each being has been created as we now see it, must occasionally have felt
surprise when he has met with an animal having habits and structure not in agreement. What can
be plainer than that the webbed feet of ducks and geese are formed for swimming? Yet there are
upland geese with webbed feet which rarely go near the water; and no one except Audubon, has
seen the frigate-bird, which has all its four toes webbed, alight on the surface of the ocean. On
the other hand, grebes and coots are eminently aquatic, although their toes are only bordered by
membrane. What seems plainer than that the long toes, not furnished with membrane, of the
Grallatores, are formed for walking over swamps and floating plants. The water-hen and landrail
are members of this order, yet the first is nearly as aquatic as the coot, and the second is nearly as
terrestrial as the quail or partridge. In such cases, and many others could be given, habits have
changed without a corresponding change of structure. The webbed feet of the upland goose may
be said to have become almost rudimentary in function, though not in structure. In the
frigate-bird, the deeply scooped membrane between the toes shows that structure has begun to
change.
He who believes in separate and innumerable acts of creation may say, that in these cases it has
pleased the Creator to cause a being of one type to take the place of one belonging to another
type; but this seems to me only restating the fact in dignified language. He who believes in the
struggle for existence and in the principle of natural selection, will acknowledge that every
organic being is constantly endeavouring to increase in numbers; and that if any one being varies
ever so little, either in habits or structure, and thus gains an advantage over some other inhabitant
of the same country, it will seize on the place of that inhabitant, however different that may be
from its own place. Hence it will cause him no surprise that there should be geese and
frigate-birds with webbed feet, living on the dry land and rarely alighting on the water, that there
should be long-toed corncrakes, living in meadows instead of in swamps; that there should be
woodpeckers where hardly a tree grows; that there should be diving thrushes and diving
Hymenoptera, and petrels with the habits of auks.
ORGANS OF EXTREME PERFECTION AND COMPLICATION.
To suppose that the eye with all its inimitable contrivances for adjusting the focus to different
distances, for admitting different amounts of light, and for the correction of spherical and
chromatic aberration, could have been formed by natural selection, seems, I freely confess,
absurd in the highest degree. When it was first said that the sun stood still and the world turned
round, the common sense of mankind declared the doctrine false; but the old saying of Vox
populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that
if numerous gradations from a simple and imperfect eye to one complex and perfect can be
shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the
eye ever varies and the variations be inherited, as is likewise certainly the case; and if such
variations should be useful to any animal under changing conditions of life, then the difficulty of
believing that a perfect and complex eye could be formed by natural selection, though
insuperable by our imagination, should not be considered as subversive of the theory. How a
nerve comes to be sensitive to light, hardly concerns us more than how life itself originated; but I
-32
may remark that, as some of the lowest organisms in which nerves cannot be detected, are
capable of perceiving light, it does not seem impossible that certain sensitive elements in their
sarcode should become aggregated and developed into nerves, endowed with this special
sensibility.
In searching for the gradations through which an organ in any species has been perfected, we
ought to look exclusively to its lineal progenitors; but this is scarcely ever possible, and we are
forced to look to other species and genera of the same group, that is to the collateral descendants
from the same parent-form, in order to see what gradations are possible, and for the chance of
some gradations having been transmitted in an unaltered or little altered condition. But the state
of the same organ in distinct classes may incidentally throw light on the steps by which it has
been perfected.
The simplest organ which can be called an eye consists of an optic nerve, surrounded by
pigment-cells and covered by translucent skin, but without any lens or other refractive body. We
may, however, according to M. Jourdain, descend even a step lower and find aggregates of
pigment-cells, apparently serving as organs of vision, without any nerves, and resting merely on
sarcodic tissue. Eyes of the above simple nature are not capable of distinct vision, and serve only
to distinguish light from darkness. In certain star-fishes, small depressions in the layer of
pigment which surrounds the nerve are filled, as described by the author just quoted, with
transparent gelatinous matter, projecting with a convex surface, like the cornea in the higher
animals. He suggests that this serves not to form an image, but only to concentrate the luminous
rays and render their perception more easy. In this concentration of the rays we gain the first and
by far the most important step towards the formation of a true, picture-forming eye; for we have
only to place the naked extremity of the optic nerve, which in some of the lower animals lies
deeply buried in the body, and in some near the surface, at the right distance from the
concentrating apparatus, and an image will be formed on it.
In the great class of the Articulata, we may start from an optic nerve simply coated with pigment,
the latter sometimes forming a sort of pupil, but destitute of lens or other optical contrivance.
With insects it is now known that the numerous facets on the cornea of their great compound
eyes form true lenses, and that the cones include curiously modified nervous filaments. But these
organs in the Articulata are so much diversified that Muller formerly made three main classes
with seven subdivisions, besides a fourth main class of aggregated simple eyes.
When we reflect on these facts, here given much too briefly, with respect to the wide,
diversified, and graduated range of structure in the eyes of the lower animals; and when we bear
in mind how small the number of all living forms must be in comparison with those which have
become extinct, the difficulty ceases to be very great in believing that natural selection may have
converted the simple apparatus of an optic nerve, coated with pigment and invested by
transparent membrane, into an optical instrument as perfect as is possessed by any member of
the Articulata class.
He who will go thus far, ought not to hesitate to go one step further, if he finds on finishing this
volume that large bodies of facts, otherwise inexplicable, can be explained by the theory of
modification through natural selection; he ought to admit that a structure even as perfect as an
eagle's eye might thus be formed, although in this case he does not know the transitional states. It
-33
has been objected that in order to modify the eye and still preserve it as a perfect instrument,
many changes would have to be effected simultaneously, which, it is assumed, could not be done
through natural selection; but as I have attempted to show in my work on the variation of
domestic animals, it is not necessary to suppose that the modifications were all simultaneous, if
they were extremely slight and gradual. Different kinds of modification would, also, serve for the
same general purpose. To arrive, however, at a just conclusion regarding the formation of the
eye, with all its marvellous yet not absolutely perfect characters, it is indispensable that the
reason should conquer the imagination; but I have felt the difficulty far to keenly to be surprised
at others hesitating to extend the principle of natural selection to so startling a length.
It is scarcely possible to avoid comparing the eye with a telescope. We know that this instrument
has been perfected by the long-continued efforts of the highest human intellects; and we
naturally infer that the eye has been formed by a somewhat analogous process. But may not this
inference be presumptuous? Have we any right to assume that the Creator works by intellectual
powers like those of man? If we must compare the eye to an optical instrument, we ought in
imagination to take a thick layer of transparent tissue, with spaces filled with fluid, and with a
nerve sensitive to light beneath, and then suppose every part of this layer to be continually
changing slowly in density, so as to separate into layers of different densities and thicknesses,
placed at different distances from each other, and with the surfaces of each layer slowly
changing in form. Further we must suppose that there is a power, represented by natural
selection or the survival of the fittest, always intently watching each slight alteration in the
transparent layers; and carefully preserving each which, under varied circumstances, in any way
or degree, tends to produce a distincter image. We must suppose each new state of the instrument
to be multiplied by the million; each to be preserved until a better is produced, and then the old
ones to be all destroyed. In living bodies, variation will cause the slight alteration, generation
will multiply them almost infinitely, and natural selection will pick out with unerring skill each
improvement. Let this process go on for millions of years; and during each year on millions of
individuals of many kinds; and may we not believe that a living optical instrument might thus be
formed as superior to one of glass, as the works of the Creator are to those of man?
If it could be demonstrated that any complex organ existed, which could not possibly have been
formed by numerous, successive, slight modifications, my theory would absolutely break down.
But we should be extremely cautious in concluding that an organ could not have been formed by
transitional gradations of some kind. Numerous cases could be given among the lower animals of
the same organ performing at the same time wholly distinct functions; thus in the larva of the
dragon-fly and in the fish Cobites the alimentary canal respires, digests, and excretes. In the
Hydra, the animal may be turned inside out, and the exterior surface will then digest and the
stomach respire. In such cases natural selection might specialise, if any advantage were thus
gained, the whole or part of an organ, which had previously performed two functions, for one
function alone, and thus by insensible steps greatly change its nature.
Again, two distinct organs, or the same organ under two very different forms, may
simultaneously perform in the same individual the same function, and this is an extremely
important means of transition: in such cases one of the two organs might readily be modified and
perfected so as to perform all the work, being aided during the progress of modification by the
other organ; and then this other organ might be modified for some other and quite distinct
purpose, or be wholly obliterated. The illustration of the swim-bladder in fishes is a good one,
-34
because it shows us clearly the highly important fact that an organ originally constructed for one
purpose, namely flotation, may be converted into one for a widely different purpose, namely
respiration. The swim-bladder has, also, been worked in as an accessory to the auditory organs of
certain fishes. All physiologists admit that the swim-bladder is homologous, or "ideally similar"
in position and structure with the lungs of the higher vertebrate animals: hence there is no reason
to doubt that the swim- bladder has actually been converted into lungs, or an organ used
exclusively for respiration.
According to this view it may be inferred that all vertebrate animals with true lungs are
descended by ordinary generation from an ancient and unknown prototype which was furnished
with a floating apparatus or swim-bladder. We can thus understand the strange fact that every
particle of food and drink which we swallow has to pass over the orifice of the trachea, with
some risk of falling into the lungs, notwithstanding the beautiful contrivance by which the glottis
is closed. In the higher Vertebrata the branchiae have wholly disappeared--but in the embryo the
slits on the sides of the neck and the loop-like course of the arteries still mark their former
position. But it is conceivable that the now utterly lost branchiae might have been gradually
worked in by natural selection for some distinct purpose: for instance, Landois has shown that
the wings of insects are developed from the trachea; it is therefore highly probable that in this
great class organs which once served for respiration have been actually converted into organs for
flight.
[An objection to the theory has been advanced on the grounds that the same organ exists in
widely diverse species in order to serve the same function, and how could this arise if the theory
is correct, for it would seem that in such widely sundered groups no part of the resemblance can
be due to inheritance from a common progenitor. But in the instances advanced the organs
achieve the same function but only superficially resemble each other; closer examination
shows–as in the distinct kinds of electric eel–that a very different organic structure is involved.]
The luminous organs which occur in a few insects, belonging to widely different families, and
which are situated in different parts of the body, offer, under our present state of ignorance, a
difficulty almost exactly parallel with that of the electric organs. Other similar cases could be
given. For instance, the eyes of Cephalopods or cuttle-fish and of vertebrate animals appear
wonderfully alike; and in such widely sundered groups no part of this resemblance can be due to
inheritance from a common progenitor. An organ for vision must be formed of transparent tissue,
and must include some sort of lens for throwing an image at the back of a darkened chamber.
Beyond this superficial resemblance, there is hardly any real similarity between the eyes of
cuttle-fish and vertebrates. Hence it is not a little difficult to decide how far even the same terms
ought to be employed in describing the eyes of the Cephalopoda and Vertebrata. It is, of course,
open to any one to deny that the eye in either case could have been developed through the natural
selection of successive slight variations; but if this be admitted in the one case it is clearly
possible in the other; and fundamental differences of structure in the visual organs of two groups
might have been anticipated, in accordance with this view of their manner of formation. As two
men have sometimes independently hit on the same invention, so in the several foregoing cases it
appears that natural selection, working for the good of each being, and taking advantage of all
favourable variations, has produced similar organs, as far as function is concerned, in distinct
organic beings, which owe none of their structure in common to inheritance from a common
progenitor.
-35
In the foregoing cases, we see the same end gained and the same function performed, in beings
not at all or only remotely allied, by organs in appearance, though not in development, closely
similar. On the other hand, it is a common rule throughout nature that the same end should be
gained, even sometimes in the case of closely related beings, by the most diversified means.
How differently constructed is the feathered wing of a bird and the membrane-covered wing of a
bat; and still more so the four wings of a butterfly, the two wings of a fly, and the two wings with
the elytra of a beetle. Bivalve shells are made to open and shut, but on what a number of patterns
is the hinge constructed, from the long row of neatly interlocking teeth in a Nucula to the simple
ligament of a Mussel! Some authors maintain that organic beings have been formed in many
ways for the sake of mere variety, almost like toys in a shop, but such a view of nature is
incredible.
I have been astonished how rarely an organ can be named, towards which no transitional grade is
known to lead. It is certainly true, that new organs appearing as if created for some special
purpose rarely or never appear in any being; as indeed is shown by that old, but somewhat
exaggerated, canon in natural history of "Natura non facit saltum." We meet with this admission
in the writings of almost every experienced naturalist; or, as Milne Edwards has well expressed
it, "Nature is prodigal in variety, but niggard in innovation." Why, on the theory of Creation,
should there be so much variety and so little real novelty? Why should all the parts and organs of
many independent beings, each supposed to have been separately created for its own proper
place in nature, be so commonly linked together by graduated steps? Why should not Nature take
a sudden leap from structure to structure? On the theory of natural selection, we can clearly
understand why she should not; for natural selection acts only by taking advantage of slight
successive variations; she can never take a great and sudden leap, but must advance by the short
and sure, though slow steps.
ORGANS OF LITTLE APPARENT IMPORTANCE, AS AFFECTED BY NATURAL
SELECTION.
As natural selection acts by life and death, by the survival of the fittest, and by the destruction of
the less well-fitted individuals, I have sometimes felt great difficulty in understanding the origin
or formation of parts of little importance; almost as great, though of a very different kind, as in
the case of the most perfect and complex organs.
In the first place, we are much too ignorant in regard to the whole economy of any one organic
being to say what slight modifications would be of importance or not. The tail of the giraffe
looks like an artificially constructed fly-flapper; and it seems at first incredible that this could
have been adapted for its present purpose by successive slight modifications, each better and
better fitted, for so trifling an object as to drive away flies; yet we should pause before being too
positive even in this case, for we know that the distribution and existence of cattle and other
animals in South America absolutely depend on their power of resisting the attacks of insects: so
that individuals which could by any means defend themselves from these small enemies, would
be able to range into new pastures and thus gain a great advantage. It is not that the larger
quadrupeds are actually destroyed (except in some rare cases) by flies, but they are incessantly
harassed and their strength reduced, so that they are more subject to disease, or not so well
enabled in a coming dearth to search for food, or to escape from beasts of prey.
-36
Organs now of trifling importance have probably in some cases been of high importance to an
early progenitor, and, after having been slowly perfected at a former period, have been
transmitted to existing species in nearly the same state, although now of very slight use. A
well-developed tail having been formed in an aquatic animal, it might subsequently come to be
worked in for all sorts of purposes, as a fly-flapper, an organ of prehension, or as an aid in
turning, as in the case of the dog, though the aid in this latter respect must be slight, for the hare,
with hardly any tail, can double still more quickly.
In the second place, we may easily err in attributing importance to characters, and in believing
that they have been developed through natural selection. For example, characters of use to one
sex are often gained and then transmitted more or less perfectly to the other sex, though of no
use to the sex. But structures thus indirectly gained, although at first of no advantage to a
species, may subsequently have been taken advantage of by its modified descendants, under new
conditions of life and newly acquired habits. The sutures in the skulls of young mammals have
been advanced as a beautiful adaptation for aiding parturition, and no doubt they facilitate, or
may be indispensable for this act; but as sutures occur in the skulls of young birds and reptiles,
which have only to escape from a broken egg, we may infer that this structure has arisen from
the laws of growth, and has been taken advantage of in the birth-process of the higher animals.
The foregoing remarks lead me to say a few words on the protest lately made by some naturalists
against the doctrine that every detail of structure has been produced for the good of its possessor.
They believe that many structures have been created for the sake of beauty, to delight man or the
Creator (but this latter point is beyond the scope of scientific discussion), or for the sake of mere
variety, a view already discussed. Such doctrines, if true, would be absolutely fatal to my theory.
I fully admit that many structures are now of no direct use to their possessors, and may never
have been of any use to their progenitors; but this does not prove that they were formed solely
for beauty or variety. We should consider, rather, that the chief part of the organisation of every
living creature is due to inheritance; and consequently, though each being assuredly is well fitted
for its place in nature, many structures have now no very close and direct relation to present
habits of life. Thus, we can hardly believe that the webbed feet of the upland goose, or of the
frigate- bird, are of special use to these birds; we cannot believe that the similar bones in the arm
of the monkey, in the fore leg of the horse, in the wing of the bat, and in the flipper of the seal,
are of special use to these animals. We may safely attribute these structures to inheritance. But
webbed feet no doubt were as useful to the progenitor of the upland goose and of the frigate-bird,
as they now are to the most aquatic of living birds. So we may believe that the progenitor of the
seal did not possess a flipper, but a foot with five toes fitted for walking or grasping; and we may
further venture to believe that the several bones in the limbs of the monkey, horse and bat, were
originally developed, on the principle of utility, probably through the reduction of more
numerous bones in the fin of some ancient fish-like progenitor of the whole class.
With respect to the belief that organic beings have been created beautiful for the delight of
man--a belief which it has been pronounced is subversive of my whole theory--I may first
remark that if beautiful objects had been created solely for man's gratification, it ought to be
shown that before man appeared there was less beauty on the face of the earth than since he
came on the stage. Were the beautiful volute and cone shells of the Eocene epoch, and the
gracefully sculptured ammonites of the Secondary period, created that man might ages
afterwards admire them in his collections? Few objects are more beautiful than the minute
-37
siliceous cases of the diatomaceae: were these created that they might be examined and admired
under the higher powers of the microscope? The beauty in this latter case, and in many others, is
apparently wholly due to symmetry of growth. Flowers rank among the most beautiful
productions of nature; but they have been rendered conspicuous in contrast with the green
leaves, and in consequence at the same time beautiful, so that they may be easily observed by
insects. I have come to this conclusion from finding it an invariable rule that when a flower is
fertilised by the wind it never has a gaily-coloured corolla.
On the other hand, I willingly admit that a great number of male animals, as all our most
gorgeous birds, some fishes, reptiles, and mammals, and a host of magnificently coloured
butterflies, have been rendered beautiful for beauty's sake. But this has been effected through
sexual selection, that is, by the more beautiful males having been continually preferred by the
females, and not for the delight of man. So it is with the music of birds. We may infer from all
this that a nearly similar taste for beautiful colours and for musical sounds runs through a large
part of the animal kingdom. When the female is as beautifully coloured as the male, which is not
rarely the case with birds and butterflies, the cause apparently lies in the colours acquired
through sexual selection having been transmitted to both sexes, instead of to the males alone.
Natural selection cannot possibly produce any modification in a species exclusively for the good
of another species; though throughout nature one species incessantly takes advantage of, and
profits by the structures of others. But natural selection can and does often produce structures for
the direct injury of other animals, as we see in the fang of the adder, and in the ovipositor of the
ichneumon, by which its eggs are deposited in the living bodies of other insects. If it could be
proved that any part of the structure of any one species had been formed for the exclusive good
of another species, it would annihilate my theory, for such could not have been produced through
natural selection. Although many statements may be found in works on natural history to this
effect, I cannot find even one which seems to me of any weight. It is admitted that the rattlesnake
has a poison-fang for its own defence and for the destruction of its prey; but some authors
suppose that at the same time it is furnished with a rattle for its own injury, namely, to warn its
prey. I would almost as soon believe that the cat curls the end of its tail when preparing to
spring, in order to warn the doomed mouse.
Natural selection tends only to make each organic being as perfect as, or slightly more perfect
than the other inhabitants of the same country with which it comes into competition. And we see
that this is the standard of perfection attained under nature. Natural selection will not produce
absolute perfection, nor do we always meet, as far as we can judge, with this high standard under
nature. If our reason leads us to admire with enthusiasm a multitude of inimitable contrivances in
nature, this same reason tells us, though we may easily err on both sides, that some other
contrivances are less perfect. Can we consider the sting of the bee as perfect, which, when used
against many kinds of enemies, cannot be withdrawn, owing to the backward serratures, and thus
inevitably causes the death of the insect by tearing out its viscera? If we look at the sting of the
bee, as having existed in a remote progenitor, as a boring and serrated instrument, like that in so
many members of the same great order, and that it has since been modified but not perfected for
its present purpose, with the poison originally adapted for some other object, such as to produce
galls, since intensified, we can perhaps understand how it is that the use of the sting should so
often cause the insect's own death: for if on the whole the power of stinging be useful to the
social community, it will fulfil all the requirements of natural selection, though it may cause the
death of some few members. If we admire the truly wonderful power of scent by which the males
-38
of many insects find their females, can we admire the production for this single purpose of
thousands of drones, which are utterly useless to the community for any other purpose, and
which are ultimately slaughtered by their industrious and sterile sisters? It may be difficult, but
we ought to admire the savage instinctive hatred of the queen-bee, which urges her to destroy the
young queens, her daughters, as soon as they are born, or to perish herself in the combat; for
undoubtedly this is for the good of the community; and maternal love or maternal hatred, though
the latter fortunately is most rare, is all the same to the inexorable principles of natural selection.
A critic has lately insisted, with some parade of mathematical accuracy, that longevity is a great
advantage to all species, so that he who believes in natural selection "must arrange his
genealogical tree" in such a manner that all the descendants have longer lives than their
progenitors! Cannot our critics conceive that a biennial plant or one of the lower animals might
range into a cold climate and perish there every winter; and yet, owing to advantages gained
through natural selection, survive from year to year by means of its seeds or ova? As for the
higher animals, longevity is generally related to the standard of each species in the scale of
organisation, as well as to the amount of expenditure in reproduction and in general activity. And
these conditions have, it is probable, been largely determined through natural selection. A more
serious objection has been urged recently, namely, that many characters appear to be of no
service whatever to their possessors, and therefore cannot have been influenced through natural
selection. Bronn adduces the length of the ears and tails in the different species of hares and
mice--the complex folds of enamel in the teeth of many animals, and a multitude of analogous
cases. There is much force in the above objection. Nevertheless, we ought to be extremely
cautious in pretending to decide what structures now are, or have formerly been, of use to each
species. It is hardly necessary to observe that even in the higher and best-known animals many
structures exist, which are so highly developed that no one doubts that they are of importance,
yet their use has not been, or has only recently been, ascertained. As the length of the ears and
tail in the several species of mice have been offered as instances, though trifling ones, of
differences in structure which can be of no special use, I may mention that the external ears of
the common mouse are supplied in an extraordinary manner with nerves, so that they no doubt
serve as tactile organs; hence the length of the ears can hardly be quite unimportant. We shall,
also, presently see that the tail is a highly useful prehensile organ to some of the species; and its
use would be much influence by its length.
A distinguished zoologist, Mr. St. George Mivart, has recently collected all the objections which
have ever been advanced by myself and others against the theory of natural selection, as
propounded by Mr. Wallace and myself, and has illustrated them with admirable art and force. A
point raised by Mr Mivart which appears to have struck many readers is, "That natural selection
is incompetent to account for the incipient stages of useful structures." I will here consider some
of the cases advanced by Mr. Mivart, beginning with the elongated neck of the giraffe, so
beautifully adapted for browsing on the higher branches of trees. Mr. Mivart brings forward
two objections. One is that the increased size of the body would obviously require an increased
supply of food, and he considers it as "very problematical whether the disadvantages thence
arising would not, in times of scarcity, more than counterbalance the advantages." But as the
giraffe does actually exist in large numbers in Africa, and as some of the largest antelopes in the
world, taller than an ox, abound there, why should we doubt that, as far as size is concerned,
intermediate gradations could formerly have existed there, subjected as now to severe dearths.
Assuredly the being able to reach, at each stage of increased size, to a supply of food, left
-39
untouched by the other hoofed quadrupeds of the country, would have been of some advantage
to the nascent giraffe. Nor must we overlook the fact, that increased bulk would act as a
protection against almost all beasts of prey excepting the lion; and against this animal, its tall
neck--and the taller the better-- would serve as a watch-tower. It is from this cause that no animal
is more difficult to stalk than the giraffe.
Mr. Mivart then asks (and this is his second objection), if natural selection be so potent, and if
high browsing be so great an advantage, why has not any other hoofed quadruped acquired a
long neck and lofty stature, besides the giraffe, and, in a lesser degree, the camel, guanaco and
macrauchenia? Or, again, why has not any member of the group acquired a long proboscis? With
respect to South Africa, which was formerly inhabited by numerous herds of the giraffe, the
answer is not difficult, and can best be given by an illustration. In every meadow in England, in
which trees grow, we see the lower branches trimmed or planed to an exact level by the
browsing of the horses or cattle; and what advantage would it be, for instance, to sheep, if kept
there, to acquire slightly longer necks? In every district some one kind of animal will almost
certainly be able to browse higher than the others; and it is almost equally certain that this one
kind alone could have its neck elongated for this purpose, through natural selection and the
effects of increased use. In South Africa the competition for browsing on the higher branches of
the acacias and other trees must be between giraffe and giraffe, and not with the other ungulate
animals. As for why, in other quarters of the world, various animals belonging to this same
order have not acquired either an elongated neck or a proboscis, that question cannot be
distinctly answered; but it is as unreasonable to expect a distinct answer to such a question as
why some event in the history of mankind did not occur in one country while it did in another.
Insects often resemble for the sake of protection various objects, such as green or decayed
leaves, dead twigs, bits of lichen, flowers, spines, excrement of birds, and living insects; but to
this latter point I shall hereafter recur. The resemblance is often wonderfully close, and is not
confined to colour, but extends to form, and even to the manner in which the insects hold
themselves. Mr. Mivart remarks, "As, according to Mr. Darwin's theory, there is a constant
tendency to indefinite variation, and as the minute incipient variations will be in all directions,
they must tend to neutralize each other, and at first to form such unstable modifications that it is
difficult, if not impossible, to see how such indefinite oscillations of infinitesimal beginnings can
ever build up a sufficiently appreciable resemblance to a leaf, bamboo, or other object, for
natural selection to seize upon and perpetuate." But the insects in their original state no doubt
presented some rude and accidental resemblance to an object commonly found in the stations
frequented by them. Nor is this at all improbable, considering the almost infinite number of
surrounding objects. Assuming that an insect originally happened to resemble in some degree a
dead twig or a decayed leaf, and that it varied slightly in many ways, then all the variations
which rendered the insect at all more like any such object, and thus favoured its escape, would
be preserved, while other variations would be neglected and ultimately lost; or, if they rendered
the insect at all less like the imitated object, they would be eliminated. There would indeed be
force in Mr. Mivart's objection, if we were to attempt to account for the above resemblances,
independently of natural selection, through mere fluctuating variability; but as the case stands
there is none.
Mr. Mivart is further inclined to believe, and some naturalists agree with him, that new species
manifest themselves "with suddenness and by modifications appearing at once." For instance, he
-40
supposes that the differences between the extinct three-toed Hipparion and the horse arose
suddenly. He thinks it difficult to believe that the wing of a bird "was developed in any other
way than by a comparatively sudden modification of a marked and important kind;" and
apparently he would extend the same view to the wings of bats and pterodactyles. But against the
belief in such abrupt changes, embryology enters a strong protest. It is notorious that the wings
of birds and bats, and the legs of horses or other quadrupeds, are undistinguishable at an early
embryonic period, and that they become differentiated by insensibly fine steps. Embryological
resemblances of all kinds can be accounted for, as we shall hereafter see, by the progenitors of
our existing species having varied after early youth, and having transmitted their newly-acquired
characters to their offspring, at a corresponding age. The embryo is thus left almost unaffected,
and serves as a record of the past condition of the species. Hence it is that existing species during
the early stages of their development so often resemble ancient and extinct forms belonging to
the same class. On this view of the meaning of embryological resemblances, and indeed on any
view, it is incredible that an animal should have undergone such momentous and abrupt
transformations as those above indicated, and yet should not bear even a trace in its embryonic
condition of any sudden modification, every detail in its structure being developed by insensibly
fine steps.
He who believes that some ancient form was transformed suddenly through an internal force or
tendency into, for instance, one furnished with wings, will be almost compelled to assume, in
opposition to all analogy, that many individuals varied simultaneously. It cannot be denied that
such abrupt and great changes of structure are widely different from those which most species
apparently have undergone. He will further be compelled to believe that many structures
beautifully adapted to all the other parts of the same creature and to the surrounding conditions,
have been suddenly produced; and of such complex and wonderful co-adaptations, he will not be
able to assign a shadow of an explanation. He will be forced to admit that these great and sudden
transformations have left no trace of their action on the embryo. To admit all this is, as it seems
to me, to enter into the realms of miracle, and to leave those of science.
Chapter eight: Instinct
OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS APPLIED TO INSTINCTS:
NEUTER AND STERILE INSECTS.
It has been objected to the foregoing view of the origin of instincts that "the variations of structure and of
instinct must have been simultaneous and accurately adjusted to each other, as a modification in the one
without an immediate corresponding change in the other would have been fatal." The force of this
objection rests entirely on the assumption that the changes in the instincts and structure are abrupt. To
take as an illustration the case of the larger titmouse, (Parus major) alluded to in a previous chapter; this
bird often holds the seeds of the yew between its feet on a branch, and hammers with its beak till it gets at
the kernel. Now what special difficulty would there be in natural selection preserving all the slight
individual variations in the shape of the beak, which were better and better adapted to break open the
seeds, until a beak was formed, as well constructed for this purpose as that of the nuthatch, at the same
time that habit, or compulsion, or spontaneous variations of taste, led the bird to become more and more
of a seed-eater? In this case the beak is supposed to be slowly modified by natural selection, subsequently
to, but in accordance with, slowly changing habits or taste; but let the feet of the titmouse vary and grow
larger from correlation with the beak, or from any other unknown cause, and it is not improbable that
such larger feet would lead the bird to climb more and more until it acquired the remarkable climbing
instinct and power of the nuthatch. In this case a gradual change of structure is supposed to lead to
-41
changed instinctive habits.
And so in other cases. It must, however, be admitted that in many instances we cannot conjecture whether
it was instinct or structure which first varied. I will not here enter on these several cases, but will confine
myself to one special difficulty, which at first appeared to me insuperable, and actually fatal to the whole
theory. I allude to the neuters or sterile females in insect communities: for these neuters often differ
widely in instinct and in structure from both the males and fertile females, and yet, from being sterile,
they cannot propagate their kind.
The subject well deserves to be discussed at great length, but I will here take only a single case, that of
working or sterile ants. How the workers have been rendered sterile is a difficulty; but not much greater
than that of any other striking modification of structure; for it can be shown that some insects and other
articulate animals in a state of nature occasionally become sterile; and if such insects had been social, and
it had been profitable to the community that a number should have been annually born capable of work,
but incapable of procreation, I can see no especial difficulty in this having been effected through natural
selection. The great difficulty lies in the working ants differing widely from both the males and the fertile
females in structure, as in the shape of the thorax, and in being destitute of wings and sometimes of eyes,
and in instinct. If a working ant or other neuter insect had been an ordinary animal, I should have
unhesitatingly assumed that all its characters had been slowly acquired through natural selection; namely,
by individuals having been born with slight profitable modifications, which were inherited by the
offspring, and that these again varied and again were selected, and so onwards. But with the working ant
we have an insect differing greatly from its parents, yet absolutely sterile; so that it could never have
transmitted successively acquired modifications of structure or instinct to its progeny. It may well be
asked how it is possible to reconcile this case with the theory of natural selection?
This difficulty, though appearing insuperable, is lessened, or, as I believe, disappears, when it is
remembered that selection may be applied to the family, as well as to the individual, and may thus gain
the desired end. Breeders of cattle wish the flesh and fat to be well marbled together. An animal thus
characterized has been slaughtered, but the breeder has gone with confidence to the same stock and has
succeeded. Such faith may be placed in the power of selection that a breed of cattle, always yielding oxen
with extraordinarily long horns, could, it is probable, be formed by carefully watching which individual
bulls and cows, when matched, produced oxen with the longest horns; and yet no one ox would ever have
propagated its kind. As with the varieties of the stock, so with social insects, selection has been applied to
the family, and not to the individual, for the sake of gaining a serviceable end. Hence, we may conclude
that slight modifications of structure or of instinct, correlated with the sterile condition of certain
members of the community, have proved advantageous; consequently the fertile males and females have
flourished, and transmitted to their fertile offspring a tendency to produce sterile members with the same
modifications. This process must have been repeated many times, until that prodigious amount of
difference between the fertile and sterile females of the same species has been produced which we see in
many social insects.
But we have not as yet touched on the acme of the difficulty; namely, the fact that the neuters of several
ants differ, not only from the fertile females and males, but from each other, sometimes to an almost
incredible degree, and are thus divided into two or even three castes. The castes, moreover, do not
generally graduate into each other, but are perfectly well defined; being as distinct from each other as are
any two species of the same genus, or rather as any two genera of the same family. [But careful study and
dissection among several species with quite distinct classes of working ants shows in all cases
intermediate grades between castes in various stages of individual workers development and rudimentary
organs where none exists to casual inspection.] With these facts before me, I believe that natural selection,
by acting on the fertile ants or parents, could form a species which should regularly produce neuters, all
of large size with one form of jaw, or all of small size with widely different jaws; or lastly, and this is the
-42
greatest difficulty, one set of workers of one size and structure, and simultaneously another set of workers
of a different size and structure; a graduated series having first been formed, as in the case of the driver
ant, and then the extreme forms having been produced in greater and greater numbers, through the
survival of the parents which generated them, until none with an intermediate structure were produced.
I have now explained how, I believe, the wonderful fact of two distinctly defined castes of sterile workers
existing in the same nest, both widely different from each other and from their parents, has originated. We
can see how useful their production may have been to a social community of ants, on the same principle
that the division of labour is useful to civilised man. Ants, however, work by inherited instincts and by
inherited organs or tools, while man works by acquired knowledge and manufactured instruments. But I
must confess, that, with all my faith in natural selection, I should never have anticipated that this principle
could have been efficient in so high a degree, had not the case of these neuter insects led me to this
conclusion. I have, therefore, discussed this case, at some little but wholly insufficient length, in order to
show the power of natural selection, and likewise because this is by far the most serious special difficulty
which my theory has encountered. The case, also, is very interesting, as it proves that with animals, as
with plants, any amount of modification may be effected by the accumulation of numerous, slight,
spontaneous variations, which are in any way profitable, without exercise or habit having been brought
into play. For peculiar habits, confined to the workers of sterile females, however long they might be
followed, could not possibly affect the males and fertile females, which alone leave descendants. I am
surprised that no one has advanced this demonstrative case of neuter insects, against the well-known
doctrine of inherited habit, as advanced by Lamarck.
Chapter fourteen: Mutual Affinities Of Organic Beings
Naturalists try to arrange the species, genera and families in each class, on what is called the Natural
System. But what is meant by this system? Some authors look at it merely as a scheme for arranging
together those living objects which are most alike, and for separating those which are most unlike; or as
an artificial method of enunciating, as briefly as possible, general propositions--that is, by one sentence to
give the characters common, for instance, to all mammals, by another those common to all carnivora, by
another those common to the dog-genus, and then, by adding a single sentence, a full description is given
of each kind of dog. The ingenuity and utility of this system are indisputable. But many naturalists think
that something more is meant by the Natural System; they believe that it reveals the plan of the Creator;
but unless it be specified whether order in time or space, or both, or what else is meant by the plan of the
Creator, it seems to me that nothing is thus added to our knowledge. Expressions such as these seem to
imply that some deeper bond is included in our classifications than mere resemblance. I believe that this is
the case, and that community of descent--the one known cause of close similarity in organic beings--is the
bond, which, though observed by various degrees of modification, is partially revealed to us by our
classifications.
Let us now consider the rules followed in classification, and the difficulties which are encountered on the
view that classification either gives some unknown plan of creation, or is simply a scheme for enunciating
general propositions and of placing together the forms most like each other. It might have been thought
(and was in ancient times thought) that those parts of the structure which determined the habits of life,
and the general place of each being in the economy of nature, would be of very high importance in
classification. Nothing can be more false. No one regards the external similarity of a mouse to a shrew, of
a dugong to a whale, of a whale to a fish, as of any importance. These resemblances, though so intimately
connected with the whole life of the being, are ranked as merely "adaptive or analogical characters;" but
to the consideration of these resemblances we shall recur. It may even be given as a general rule, that the
less any part of the organisation is concerned with special habits, the more important it becomes for
classification.
-43
That the physiological importance of an organ does not determine its classificatory value, is almost
proved by the fact, that in allied groups, in which the same organ, as we have every reason to suppose,
has nearly the same physiological value, its classificatory value is widely different. [E.g., the eye of the
cephalopod and the mammalian eye do not, for all their functional and degree of structural resemblance,
indicate that the organisms possessing them belong to the same class. And by the same token,] no one
will say that rudimentary or atrophied organs are of high physiological or vital importance; yet,
undoubtedly, organs in this condition are often of much value in classification. No one will dispute that
the rudimentary teeth in the upper jaws of young ruminants, and certain rudimentary bones of the leg, are
highly serviceable in exhibiting the close affinity between Ruminants and Pachyderms.
Numerous instances could be given of characters derived from parts which must be considered of very
trifling physiological importance, but which are universally admitted as highly serviceable in the
definition of whole groups. [Further,] we can see why characters derived from the embryo should be of
equal importance with those derived from the adult, for a natural classification of course includes all ages.
But it is by no means obvious, on the ordinary view, why the structure of the embryo should be more
important for this purpose than that of the adult, which alone plays its full part in the economy of nature.
But there can be no doubt that embryonic, excluding larval characters, are of the highest value for
classification, not only with animals but with plants. Thus the main divisions of flowering plants are
founded on differences in the embryo--on the number and position of the cotyledons, and on the mode of
development of the plumule and radicle. We shall immediately see why these characters possess so high a
value in classification, namely, from the natural system being genealogical in its arrangement.
It may be worth while to illustrate this view of classification, by taking the case of languages. If we
possessed a perfect pedigree of mankind, a genealogical arrangement of the races of man would afford the
best classification of the various languages now spoken throughout the world; and if all extinct languages,
and all intermediate and slowly changing dialects, were to be included, such an arrangement would be the
only possible one. Yet it might be that some ancient languages had altered very little and had given rise to
few new languages, whilst others had altered much owing to the spreading, isolation and state of
civilisation of the several co-descended races, and had thus given rise to many new dialects and
languages. The various degrees of difference between the languages of the same stock would have to be
expressed by groups subordinate to groups; but the proper or even the only possible arrangement would
still be genealogical; and this would be strictly natural, as it would connect together all languages, extinct
and recent, by the closest affinities, and would give the filiation and origin of each tongue.
We can understand, on these views, the very important distinction between real affinities and analogical
or adaptive resemblances. Lamarck first called attention to this subject, and he has been ably followed by
Macleay and others. The resemblance in the shape of the body and in the fin-like anterior limbs between
dugongs and whales, and between these two orders of mammals and fishes, are analogical. So is the
resemblance between a mouse and a shrew-mouse (Sorex), which belong to different orders. Among
insects there are innumerable instances; thus Linnaeus, misled by external appearances, actually classed
an homopterous insect as a moth. The resemblance between the greyhound and race-horse is hardly more
fanciful than the analogies which have been drawn by some authors between widely different animals.
On the view of characters being of real importance for classification, only in so far as they reveal descent,
we can clearly understand why analogical or adaptive characters, although of the utmost importance to the
welfare of the being, are almost valueless to the systematist. For animals, belonging to two most distinct
lines of descent, may have become adapted to similar conditions, and thus have assumed a close external
resemblance; but such resemblances will not reveal--will rather tend to conceal their blood-relationship.
We can thus also understand the apparent paradox, that the very same characters are analogical when one
group is compared with another, but give true affinities when the members of the same group are
compared together: thus the shape of the body and fin-like limbs are only analogical when whales are
-44
compared with fishes, being adaptations in both classes for swimming through the water; but between the
the several members of the whale family, the shape of the body and the fin-like limbs offer characters
exhibiting true affinity; for as these parts are so nearly similar throughout the whole family, we cannot
doubt that they have been inherited from a common ancestor. So it is with fishes.
We have seen that the members of the same class, independently of their habits of life, resemble each
other in the general plan of their organisation. This resemblance is often expressed by the term "unity of
type;" or by saying that the several parts and organs in the different species of the class are homologous.
The whole subject is included under the general term of Morphology. This is one of the most interesting
departments of natural history, and may almost be said to be its very soul. What can be more curious than
that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of
the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include
similar bones, in the same relative positions? How curious it is, to give a subordinate though striking
instance, that the hind feet of the kangaroo, which are so well fitted for bounding over the open
plains--those of the climbing, leaf-eating koala, equally well fitted for grasping the branches of
trees--those of the ground-dwelling, insect or root-eating, bandicoots--and those of some other Australian
marsupials-- should all be constructed on the same extraordinary type, namely with the bones of the
second and third digits extremely slender and enveloped within the same skin, so that they appear like a
single toe furnished with two claws. Notwithstanding this similarity of pattern, it is obvious that the hind
feet of these several animals are used for as widely different purposes as it is possible to conceive.
Geoffroy St. Hilaire has strongly insisted on the high importance of relative position or connexion in
homologous parts; they may differ to almost any extent in form and size, and yet remain connected
together in the same invariable order. We never find, for instance, the bones of the arm and forearm, or of
the thigh and leg, transposed. Hence the same names can be given to the homologous bones in widely
different animals. We see the same great law in the construction of the mouths of insects: what can be
more different than the immensely long spiral proboscis of a sphinx-moth, the curious folded one of a bee
or bug, and the great jaws of a beetle? Yet all these organs, serving for such widely different purposes, are
formed by infinitely numerous modifications of an upper lip, mandibles, and two pairs of maxillae. The
same law governs the construction of the mouths and limbs of crustaceans. So it is with the flowers of
plants.
Nothing can be more hopeless than to attempt to explain this similarity of pattern in members of the same
class, by utility or by the doctrine of final causes. The hopelessness of the attempt has been expressly
admitted by Owen in his most interesting work on the "Nature of Limbs." On the ordinary view of the
independent creation of each being, we can only say that so it is; that it has pleased the Creator to
construct all the animals and plants in each great class on a uniform plan; but this is not a scientific
explanation.
The explanation is to a large extent simple, on the theory of the selection of successive slight
modifications, each being profitable in some way to the modified form, but often affecting by correlation
other parts of the organisation. In changes of this nature, there will be little or no tendency to alter the
original pattern, or to transpose the parts. The bones of a limb might be shortened and flattened to any
extent, becoming at the same time enveloped in thick membrane, so as to serve as a fin; or a webbed hand
might have all its bones, or certain bones, lengthened to any extent, with the membrane connecting them
increased, so as to serve as a wing; yet all these modifications would not tend to alter the framework of
the bones or the relative connexion of the parts. If we suppose that an early progenitor--the archetype, as
it may be called--of all mammals, birds and reptiles, had its limbs constructed on the existing general
pattern, for whatever purpose they served, we can at once perceive the plain signification of the
homologous construction of the limbs throughout the class. So with the mouths of insects, we have only
to suppose that their common progenitor had an upper lip, mandibles, and two pairs of maxillae, these
-45
parts being perhaps very simple in form; and then natural selection will account for the infinite diversity
in structure and function of the mouths of insects. Nevertheless, it is conceivable that the general pattern
of an organ might become so much obscured as to be finally lost, by the reduction and ultimately by the
complete abortion of certain parts, by the fusion of other parts, and by the doubling or multiplication of
others, variations which we know to be within the limits of possibility. In the paddles of the gigantic
extinct sea-lizards, and in the mouths of certain suctorial crustaceans, the general pattern seems thus to
have become partially obscured.
How inexplicable are the cases of serial homologies on the ordinary view of creation! Why should the
brain be enclosed in a box composed of such numerous and such extraordinarily shaped pieces of bone
apparently representing vertebrae? As Owen has remarked, the benefit derived from the yielding of the
separate pieces in the act of parturition by mammals, will by no means explain the same construction in
the skulls of birds and reptiles. Why should similar bones have been created to form the wing and the leg
of a bat, used as they are for such totally different purposes, namely flying and walking? Why should one
crustacean, which has an extremely complex mouth formed of many parts, consequently always have
fewer legs; or conversely, those with many legs have simpler mouths? Why should the sepals, petals,
stamens, and pistils, in each flower, though fitted for such distinct purposes, be all constructed on the
same pattern?
On the theory of natural selection, we can, to a certain extent, answer these questions. Naturalists
frequently speak of the skull as formed of metamorphosed vertebrae; the jaws of crabs as metamorphosed
legs; the stamens and pistils in flowers as metamorphosed leaves. ; but it would in most cases be more
correct, as Professor Huxley has remarked, to speak of both skull and vertebrae, jaws and legs, etc., as
having been metamorphosed, not one from the other, as they now exist, but from some common and
simpler element. Most naturalists use such language only in a metaphorical sense: they are far from
meaning that during a long course of descent, primordial organs of any kind--vertebrae in the one case
and legs in the other--have actually been converted into skulls or jaws. Yet so strong is the appearance of
this having occurred that naturalists can hardly avoid employing language having this plain signification.
According to the views here maintained, such language may be used literally; and the wonderful fact of
the jaws, for instance, of a crab retaining numerous characters, which they probably would have retained
through inheritance, if they had really been metamorphosed from true though extremely simple legs, is in
part explained.
Chapter fifteen: Recapitulation And Conclusion
As this whole volume is one long argument, it may be convenient to the reader to have the leading facts
and inferences briefly recapitulated.
That many and serious objections may be advanced against the theory of descent with modification
through variation and natural selection, I do not deny. I have endeavoured to give to them their full force.
Nothing at first can appear more difficult to believe than that the more complex organs and instincts have
been perfected, not by means superior to, though analogous with, human reason, but by the accumulation
of innumerable slight variations, each good for the individual possessor. Nevertheless, this difficulty,
though appearing to our imagination insuperably great, cannot be considered real if we admit the
following propositions, namely, that all parts of the organisation and instincts offer, at least individual
differences--that there is a struggle for existence leading to the preservation of profitable deviations of
structure or instinct--and, lastly, that gradations in the state of perfection of each organ may have existed,
each good of its kind. The truth of these propositions cannot, I think, be disputed.
Now let us turn to the other side of the argument. Under domestication we see much variability, caused,
or at least excited, by changed conditions of life; but often in so obscure a manner, that we are tempted to
-46
consider the variations as spontaneous. Variability is not actually caused by man; he only unintentionally
exposes organic beings to new conditions of life and then nature acts on the organisation and causes it to
vary. But man can and does select the variations given to him by nature, and thus accumulates them in
any desired manner. He thus adapts animals and plants for his own benefit or pleasure. He may do this
methodically, or he may do it unconsciously by preserving the individuals most useful or pleasing to him
without any intention of altering the breed. It is certain that he can largely influence the character of a
breed by selecting, in each successive generation, individual differences so slight as to be inappreciable
except by an educated eye. This unconscious process of selection has been the great agency in the
formation of the most distinct and useful domestic breeds. That many breeds produced by man have to a
large extent the character of natural species, is shown by the inextricable doubts whether many of them
are varieties or aboriginally distinct species.
There is no reason why the principles which have acted so efficiently under domestication should not
have acted under nature. In the survival of favoured individuals and races, during the constantly recurrent
Struggle for Existence, we see a powerful and ever-acting form of Selection. The struggle for existence
inevitably follows from the high geometrical ratio of increase which is common to all organic beings.
This high rate of increase is proved by calculation--by the rapid increase of many animals and plants
during a succession of peculiar seasons, and when naturalised in new countries. More individuals are born
than can possibly survive. A grain in the balance may determine which individuals shall live and which
shall die--which variety or species shall increase in number, and which shall decrease, or finally become
extinct. As the individuals of the same species come in all respects into the closest competition with each
other, the struggle will generally be most severe between them; it will be almost equally severe between
the varieties of the same species, and next in severity between the species of the same genus. On the other
hand the struggle will often be severe between beings remote in the scale of nature. The slightest
advantage in certain individuals, at any age or during any season, over those with which they come into
competition, or better adaptation in however slight a degree to the surrounding physical conditions, will,
in the long run, turn the balance.
As geology plainly proclaims that each land has undergone great physical changes, we might have
expected to find that organic beings have varied under nature, in the same way as they have varied under
domestication. And if there has been any variability under nature, it would be an unaccountable fact if
natural selection had not come into play. It has often been asserted, but the assertion is incapable of proof,
that the amount of variation under nature is a strictly limited quantity. If, then, animals and plants do
vary, let it be ever so slightly or slowly, why should not variations or individual differences, which are in
any way beneficial, be preserved and accumulated through natural selection, or the survival of the fittest?
On the view that species are only strongly marked and permanent varieties, and that each species first
existed as a variety, we can see why it is that no line of demarcation can be drawn between species,
commonly supposed to have been produced by special acts of creation, and varieties which are
acknowledged to have been produced by secondary laws. Moreover, the species of the larger genera,
which afford the greater number of varieties or incipient species, retain to a certain degree the character of
varieties; for they differ from each other by a less amount of difference than do the species of smaller
genera. The closely allied species also of a larger genera apparently have restricted ranges, and in their
affinities they are clustered in little groups round other species--in both respects resembling varieties.
These are strange relations on the view that each species was independently created, but are intelligible if
each existed first as a variety.
As natural selection acts solely by accumulating slight, successive, favourable variations, it can produce
no great or sudden modifications; it can act only by short and slow steps. Hence, the canon of "Natura
non facit saltum," which every fresh addition to our knowledge tends to confirm, is on this theory
intelligible. We can see why throughout nature the same general end is gained by an almost infinite
-47
diversity of means, for every peculiarity when once acquired is long inherited, and structures already
modified in many different ways have to be adapted for the same general purpose. We can, in short, see
why nature is prodigal in variety, though niggard in innovation. But why this should be a law of nature if
each species has been independently created no man can explain.
As natural selection acts by competition, it adapts and improves the inhabitants of each country only in
relation to their co-inhabitants; so that we need feel no surprise at the species of any one country,
although on the ordinary view supposed to have been created and specially adapted for that country,
being beaten and supplanted by the naturalised productions from another land. Nor ought we to marvel if
all the contrivances in nature be not, as far as we can judge, absolutely perfect; as in the case even of the
human eye; or if some of them be abhorrent to our ideas of fitness. We need not marvel at the sting of the
bee, when used against the enemy, causing the bee's own death; at drones being produced in such great
numbers for one single act, and being then slaughtered by their sterile sisters; at the astonishing waste of
pollen by our fir-trees; at the instinctive hatred of the queen-bee for her own fertile daughters; at
ichneumonidae feeding within the living bodies of caterpillars; and at other such cases. The wonder,
indeed, is, on the theory of natural selection, that more cases of the want of absolute perfection have not
been detected.
If we admit that the geological record is imperfect to an extreme degree, then the facts, which the record
does give, strongly support the theory of descent with modification. New species have come on the stage
slowly and at successive intervals; and the amount of change after equal intervals of time, is widely
different in different groups. The extinction of species and of whole groups of species, which has played
so conspicuous a part in the history of the organic world, almost inevitably follows from the principle of
natural selection; for old forms are supplanted by new and improved forms. Neither single species nor
groups of species reappear when the chain of ordinary generation is once broken. The gradual diffusion of
dominant forms, with the slow modification of their descendants, causes the forms of life, after long
intervals of time, to appear as if they had changed simultaneously throughout the world. The fact of the
fossil remains of each formation being in some degree intermediate in character between the fossils in the
formations above and below, is simply explained by their intermediate position in the chain of descent.
The grand fact that all extinct beings can be classed with all recent beings, naturally follows from the
living and the extinct being the offspring of common parents. As species have generally diverged in
character during their long course of descent and modification, we can understand why it is that the more
ancient forms, or early progenitors of each group, so often occupy a position in some degree intermediate
between existing groups. Recent forms are generally looked upon as being, on the whole, higher in the
scale of organisation than ancient forms; and they must be higher, in so far as the later and more improved
forms have conquered the older and less improved forms in the struggle for life; they have also generally
had their organs more specialised for different functions. This fact is perfectly compatible with numerous
beings still retaining simple and but little improved structures, fitted for simple conditions of life; it is
likewise compatible with some forms having retrograded in organisation, by having become at each stage
of descent better fitted for new and degraded habits of life. Lastly, the wonderful law of the long
endurance of allied forms on the same continent--of marsupials in Australia, of edentata in America, and
other such cases--is intelligible, for within the same country the existing and the extinct will be closely
allied by descent.
The similar framework of bones in the hand of a man, wing of a bat, fin of the porpoise, and leg of the
horse--the same number of vertebrae forming the neck of the giraffe and of the elephant--and innumerable
other such facts, at once explain themselves on the theory of descent with slow and slight successive
modifications. The similarity of pattern in the wing and in the leg of a bat, though used for such different
purpose--in the jaws and legs of a crab--in the petals, stamens, and pistils of a flower, is likewise, to a
large extent, intelligible on the view of the gradual modification of parts or organs, which were
aboriginally alike in an early progenitor in each of these classes. On the principle of successive variations
-48
not always supervening at an early age, and being inherited at a corresponding not early period of life, we
clearly see why the embryos of mammals, birds, reptiles, and fishes should be so closely similar, and so
unlike the adult forms. We may cease marvelling at the embryo of an air-breathing mammal or bird
having branchial slits and arteries running in loops, like those of a fish which has to breathe the air
dissolved in water by the aid of well-developed branchiae.
It can hardly be supposed that a false theory would explain, in so satisfactory a manner as does the theory
of natural selection, the several large classes of facts above specified. It has recently been objected that
this is an unsafe method of arguing; but it is a method used in judging of the common events of life, and
has often been used by the greatest natural philosophers. The undulatory theory of light has thus been
arrived at; and the belief in the revolution of the earth on its own axis was until lately supported by hardly
any direct evidence. It is no valid objection that science as yet throws no light on the far higher problem
of the essence or origin of life. Who can explain what is the essence of the attraction of gravity? No one
now objects to following out the results consequent on this unknown element of attraction;
notwithstanding that Leibnitz formerly accused Newton of introducing "occult qualities and miracles into
philosophy."
I see no good reasons why the views given in this volume should shock the religious feelings of any one.
It is satisfactory, as showing how transient such impressions are, to remember that the greatest discovery
ever made by man, namely, the law of the attraction of gravity, was also attacked by Leibnitz, "as
subversive of natural, and inferentially of revealed, religion." A celebrated author and divine has written
to me that "he has gradually learned to see that it is just as noble a conception of the Deity to believe that
He created a few original forms capable of self- development into other and needful forms, as to believe
that He required a fresh act of creation to supply the voids caused by the action of His laws."
It may be asked how far I extend the doctrine of the modification of species. The question is difficult to
answer, because the more distinct the forms are which we consider, by so much the arguments in favour
of community of descent become fewer in number and less in force. But some arguments of the greatest
weight extend very far. All the members of whole classes are connected together by a chain of affinities,
and all can be classed on the same principle, in groups subordinate to groups. Fossil remains sometimes
tend to fill up very wide intervals between existing orders. Organs in a rudimentary condition plainly
show that an early progenitor had the organ in a fully developed condition, and this in some cases implies
an enormous amount of modification in the descendants. Throughout whole classes various structures are
formed on the same pattern, and at a very early age the embryos closely resemble each other. Therefore I
cannot doubt that the theory of descent with modification embraces all the members of the same great
class or kingdom. I believe that animals are descended from at most only four or five progenitors, and
plants from an equal or lesser number.
Analogy would lead me one step further, namely, to the belief that all animals and plants are descended
from some one prototype. But analogy may be a deceitful guide. Nevertheless all living things have much
in common, in their chemical composition, their cellular structure, their laws of growth, and their liability
to injurious influences. We see this even in so trifling a fact as that the same poison often similarly affects
plants and animals; or that the poison secreted by the gall-fly produces monstrous growths on the wild
rose or oak-tree. With all organic beings, excepting perhaps some of the very lowest, sexual reproduction
seems to be essentially similar. With all, as far as is at present known, the germinal vesicle is the same; so
that all organisms start from a common origin. If we look even to the two main divisions--namely, to the
animal and vegetable kingdoms--certain low forms are so far intermediate in character that naturalists
have disputed to which kingdom they should be referred.
When the views advanced by me in this volume, and by Mr. Wallace or when analogous views on the
origin of species are generally admitted, we can dimly foresee that there will be a considerable revolution
-49
in natural history. Systematists will be able to pursue their labours as at present; but they will not be
incessantly haunted by the shadowy doubt whether this or that form be a true species. This, I feel sure and
I speak after experience, will be no slight relief. Hereafter we shall be compelled to acknowledge that the
only distinction between species and well-marked varieties is, that the latter are known, or believed to be
connected at the present day by intermediate gradations, whereas species were formerly thus connected.
Hence, without rejecting the consideration of the present existence of intermediate gradations between
any two forms, we shall be led to weigh more carefully and to value higher the actual amount of
difference between them. In short, we shall have to treat species in the same manner as those naturalists
treat genera, who admit that genera are merely artificial combinations made for convenience. This may
not be a cheering prospect; but we shall at least be freed from the vain search for the undiscovered and
undiscoverable essence of the term species.
The other and more general departments of natural history will rise greatly in interest. The terms used by
naturalists, of affinity, relationship, community of type, paternity, morphology, adaptive characters,
rudimentary and aborted organs, etc., will cease to be metaphorical and will have a plain signification.
When we no longer look at an organic being as a savage looks at a ship, as something wholly beyond his
comprehension; when we regard every production of nature as one which has had a long history; when we
contemplate every complex structure and instinct as the summing up of many contrivances, each useful to
the possessor, in the same way as any great mechanical invention is the summing up of the labour, the
experience, the reason, and even the blunders of numerous workmen; when we thus view each organic
being, how far more interesting--I speak from experience--does the study of natural history become!
A grand and almost untrodden field of inquiry will be opened, on the causes and laws of variation, on
correlation, on the effects of use and disuse, on the direct action of external conditions, and so forth. And
I see open fields for far more important researches. Psychology will be securely based on the foundation
already well laid by Mr. Herbert Spencer, that of the necessary acquirement of each mental power and
capacity by gradation. Much light will be thrown on the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with the view that each species has been
independently created. To my mind it accords better with what we know of the laws impressed on matter
by the Creator, that the production and extinction of the past and present inhabitants of the world should
have been due to secondary causes, like those determining the birth and death of the individual. When I
view all beings not as special creations, but as the lineal descendants of some few beings which lived long
before the first bed of the Cambrian system was deposited, they seem to me to become ennobled. Judging
from the past, we may safely infer that not one living species will transmit its unaltered likeness to a
distinct futurity. And of the species now living very few will transmit progeny of any kind to a far distant
futurity; for the manner in which all organic beings are grouped, shows that the greater number of species
in each genus, and all the species in many genera, have left no descendants, but have become utterly
extinct. We can so far take a prophetic glance into futurity as to foretell that it will be the common and
widely spread species, belonging to the larger and dominant groups within each class, which will
ultimately prevail and procreate new and dominant species. As all the living forms of life are the lineal
descendants of those which lived long before the Cambrian epoch, we may feel certain that the ordinary
succession by generation has never once been broken, and that no cataclysm has desolated the whole
world. Hence, we may look with some confidence to a secure future of great length. And as natural
selection works solely by and for the good of each being, all corporeal and mental endowments will tend
to progress towards perfection.
It is interesting to contemplate a tangled bank, clothed with many plants of many kinds, with birds singing
on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and
to reflect that these elaborately constructed forms, so different from each other, and dependent upon each
other in so complex a manner, have all been produced by laws acting around us. These laws, taken in the
-50
largest sense, being Growth with reproduction; Inheritance which is almost implied by reproduction;
Variability from the indirect and direct action of the conditions of life, and from use and disuse; a Ratio of
Increase so high as to lead to a Struggle for Life, and as a consequence to Natural Selection, entailing
Divergence of Character and the Extinction of less improved forms. Thus, from the war of nature, from
famine and death, the most exalted object which we are capable of conceiving, namely, the production of
the higher animals, directly follows. There is grandeur in this view of life, with its several powers, having
been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone
circling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful
and most wonderful have been, and are being evolved.
-51